Google originally developed the OS and turned it over to the Open Handset Alliance. It allows developers to write managed code in the Java language, controlling the device via Google-developed Java libraries.
This is an open source operating system that focuses on the Internet exploiting features like cloud computing. It runs on ARM, MIPS as well as X86 processors. Developers can use the Android + Tegra powered SDK for Nvidia graphics hardware. The various programs that run on this OS are generally called apps.
 OS details
The design of the original Unix was a break through in that the various pieces were isolated from each other in layers. The basic kernel is the heart of the OS. The layer above, called the shell, contains all of the interaction with the user. The kernel communication was designed about libraries of specialized code routines that is linked to the shell program or in some cases used directly from an application. When a graphics interface was designed it became yet an additional layer. This isolation provided both well defined boundaries and security. Android took advantage of this design and built their OS on top of a Linux kernel which made it a completely different OS. Software is generally not compatible between the two OS. The Android user space it built around Java.
Unfortunately many of the hardware releases are not supported with updates so there is a wide variety of versions in the customer base. There seems to be a split in V2 and V3 where V2 continues to be developed for small screen devices such as smartphones and small tablets up to about 7" screens and V3 is targeted at larger screen Web Tablets. The code names are in alphabetical order (Cupcake, Donut, Eclair, Froyo, Gingerbread, Honeycomb, Ice Cream Sandwich, Jelly Bean, Kit-Kat, Lollipop, Marshmallow, Nougat, and Oreo.
 Cupcake (V1.5)
April 30, 2009 marked the official Cupcake release.
- Virtual keyboard: Support for 3rd party keyboards with text prediction & user dictionary for custom words
- Widgets: Are miniature application views that can be embedded in other applications (such as the Home screen) and receive periodic updates
- Camera: Video recording
- Gallery: Video playback (MPEG-4 & 3GP formats)
- Bluetooth: Stereo support added (A2DP and AVRCP profiles), Auto-pairing
- Browser: Copy and paste features added
- Contacts: Shows user picture for Favorites
- Dialer: Specific date/time stamp for events in call log and one-touch access to a contact card from call log event
- System: Animated screen transitions
- Upload videos to YouTube
- Upload photos on Picasa
 Donut (V1.6)
September 15, 2009 Donut SDK was released.
- Search: Voice Search & text entry search enhanced to include bookmarks & history, contacts, the web, and more
- Search: Developers can now include their content in search results
- Text to speech: Features a multi-lingual speech synthesis engine to allow any Android application to "speak" a string of text
- Android Market: Allows easier searching, app screenshots, etc.
- Camera, camcorder, and Gallery: Updated integrated with faster camera access
- Gallery: Now enables users to select multiple photos for deletion
- System: Updated technology support for CDMA/EVDO, 802.1x, VPNs, and a text-to-speech engine
- Display: Support for WVGA screen resolutions
- Speed improvements in searching and camera applications
- Expanded Gesture framework and new GestureBuilder development tool
- Google free turn-by-turn navigation
 Eclair (V2.0 and 2.1)
October 26, 2009 Eclair 2.0 was released.
- Sync: Expanded Account sync. Multiple accounts can be added to a device for email and contact synchronization
- Email: Exchange support, Combined inbox to browse email from multiple accounts in one page.
- Bluetooth: 2.1 support
- Contacts: Tap a contact photo and select to call, SMS, or email the person.
- Messaging: Search all saved SMS and MMS messages. Auto delete oldest messages in a conversation when a defined limit is reached.
- Camera: Flash support, Digital zoom, Scene mode, White balance, Color effect, Macro focus
- Virtual keyboard: Improved typing speed, smarter dictionary learns from word usage and includes contact names as suggestions.
- Browser: Refreshed UI, Bookmark thumbnails, Double-tap zoom, Support for HTML5
- Calendar: Agenda view enhanced, Attending status for each invitee, Invite new guests to events.
- System: Optimized hardware speed, Revamped UI
- Display: Support for more screen sizes and resolutions, Better contrast ratio
- Maps: Improved Google Maps 3.1.2
- MotionEvent class enhanced to track multi-touch events
- Live Wallpapers: Home screen background images can be animated to show movement
V2.1 was API changes and bug fixes and was released on January 12, 2010
 Froyo (V2.2)
Android 2.2 offers a handful of new features and benefits such as faster web browsing, simple app launching and updating, and a completely more intuitive experience through Google's impressive services.
- Better overall performance of Android OS
- USB and Wi-Fi Internet Tethering
- Microsoft Exchange integration
- New and improved browser
- Compass support for Google Maps
- Improved Google Voice Search, recognizes long and complicated
- High Density screen support such as 720p on a 4" screen.
 Gingerbread (V2.3)
Released December 6, 2010 and is the current version as of July 2011. It has more intuitive text input and better power management.
- Display: Support for extra-large screen sizes and resolutions (WXGA and higher)
- Internet calling: Native support for SIP VoIP telephony
- Virtual Keyboard: Faster, more intuitive text input, improved accuracy, better suggested text.
- Voice input mode
- Copy/Paste: Enhanced. Select a word by press-hold, copy, and paste
- Near Field Communication lets the user read an NFC tag embedded in a poster, sticker, or advertisement
- New audio effects such as reverb, equalization, headphone virtualization, and bass boost
- New Download Manager giving users easy access to any file downloaded from the browser, email, or another application
- Camera: Access multiple cameras on the device, including a front-facing camera, if available
- Media: Support for WebM/VP8 video playback, and AAC audio encoding
- System: Improved power management with a more active role in managing apps that are keeping the device awake for too long
- System: Enhanced support for native code development
- System: Switched from YAFFS to ext4 on newer devices
- Audio, graphical, and input enhancements for game developers
- Concurrent garbage collection for increased performance
- Native support for more sensors (such as gyroscopes and barometers)
 Honeycomb (V3.0)
Honeycomb (version 3.0), just like other versions of Android, is a modern Linux-based OS which uses the Dalvik virtual machine to run code similar to Java apps. It supports multitasking and just-in-time compilation, and generally has good memory management. It is designed specifically for touchscreen Web Tablets. It was released on February 22, 2011
- Optimized tablet support with a new virtual and “holographic” user interface
- System Bar: Quick access to notifications, status, and soft navigation buttons available at the bottom of the screen
- Action Bar: Access to contextual options, navigation, widgets, or other types of content at the top of the screen
- Multitasking: Tap Recent Apps in the System Bar, to see snapshots of the tasks underway and quickly jump from one app to another
- Redesigned keyboard: To make entering text fast and accurate on larger screen sizes with greater accuracy and efficiency
- Copy/Paste: Simplified, more intuitive
- Browser: Multiple tabs replace browser windows, form auto-fill, and a new “incognito” mode allows anonymous browsing
- Camera: Quick access to exposure, focus, flash, zoom, front-facing camera, time-lapse, and more
- Gallery: View albums and other collections in full-screen mode, with easy access to thumbnails for other photos
- Contacts: New two-pane UI and Fast Scroll to let users easily organize and locate contacts
- Email: New two-pane UI to make viewing and organizing messages more efficient. The app lets users select one or more messages
- Support for video chat using Google Talk
- Hardware acceleration
- Support for multi-core processors
This version was released May 10, 2011 and is the current version as of July 2011. It adds a few user interface refinements and USB connectivity for accessories such as keyboards, mice, joysticks, and gamepads. It also improves the Wi-Fi robustness and updates many of the standard applications such as the browser.
- UI refinements
- Connectivity for USB accessories (Host mode) AKA OTG using adapters.
- Expanded Recent Apps list
- Resizable Home screen widgets
- Support for external keyboards and pointing devices
- Support for joysticks and gamepads
- Media: Support for FLAC
- High-performance Wi-Fi lock maintains high-performance Wi-Fi connections when device screen is off
- Support for HTTP proxy for each connected Wi-Fi access point
Released July 15, 2011
- Improved hardware support, including optimizations for a wider range of tablets
- Easier access for apps to files on the SD card, e.g. for synchronization
- Compatibility display mode for apps that have not yet been optimized for tablet screen resolutions
- New display support functions that give developers more control over the look and feel on different Android devices.
 Ice Cream Sandwich (4.0)
Released OCT 2011. Designed to for integrate Honeycomb and Gingerbread.
- Major refinements to the "Holo" interface with new Roboto font family.
- Soft buttons from Android 3.x are now available for use on phones.
- Separation of widgets in a new tab, listed in a similar manner to applications
- Easier-to-create folders, with a drag-and-drop style
- A customizable launcher
- Improved visual voicemail with the ability to speed up or slow down voicemail messages
- Pinch-to-zoom functionality in the calendar
- Integrated screenshot capture (accomplished by holding down the Power and Volume-Down buttons)
- Improved error correction on the keyboard
- Ability to access applications directly from lock screen
- Improved copy and paste functionality
- Better voice integration and continuous, real-time speech to text dictation
- Face Unlock, a feature that allows users to unlock handsets using facial recognition software
- Automatic syncing of browser with users' Chrome bookmarks
- Data Usage section in settings that lets users set warnings when they approach a certain usage limit, and disable data use when the limit is exceeded
- Ability to shut down applications that are using data in the background
- Improved camera application with zero shutter lag, time lapse settings, panorama mode, and the ability to zoom while recording
- Built-in photo editor
- New gallery layout, organized by location and person
- Refreshed "People" application with social network integration, status updates and hi-res images
- Android Beam, a near-field communication (NFC) feature allowing the rapid short-range exchange of web bookmarks, contact info, directions, YouTube videos and other data
- Support for the WebP image format
- Hardware acceleration of the UI
- Wi-Fi Direct
- 1080p video recording for stock Android devices
- Android VPN Framework (AVF), and TUN (but not TAP) kernel module. Prior to 4.0, VPN software required rooted Android.
 Jelly Bean (V4.1)
- Vsync timing across all drawing and animation done by the Android framework, including application rendering, touch events, screen composition and display refresh
- Triple buffering in the graphics pipeline
- Enhanced accessibility
- Bi-directional text and other language support
- User-installable keyboard maps
- Expandable notifications
- Ability to turn off notifications on an application-specific basis
- Shortcuts and widgets can automatically be re-arranged or re-sized to allow new items to fit on home screens
- Bluetooth data transfer for Android Beam (NFC)
- Offline voice dictation
- Tablets with smaller screens now use an expanded version of the interface layout and home screen used by phones.
- Improved voice search
- Improved camera application
- Google Wallet (for the Nexus 7)
- High-resolution Google+ contact photos
- Google Now voice assistant and search application
- Multichannel audio
- USB audio (for external sound DACs)
- Audio chaining (also known as gapless playback)
- Stock Android browser is replaced with the Android mobile version of Google Chrome in devices with Android 4.1 preinstalled
- Ability for other launchers to add widgets from the application drawer without requiring root access
- Lock/home screen rotation support for the Nexus 7
- One-finger gestures to expand/collapse notifications
- Lock screen improvements, including widget support and the ability to swipe directly to camera
- Notification power controls ("Quick Settings")
- "Daydream" screensavers, showing information when idle or docked
- Multiple user accounts (tablets only)
- Support for wireless display (Miracast)
- Accessibility improvements: triple-tap to magnify the entire screen, pan and zoom with two fingers. Speech output and Gesture Mode navigation for blind users
- New clock application with built-in world clock, stop watch and timer
- All devices now use the same interface layout, previously adapted from phones on 4.1 for smaller tablets (with centered software buttons, the system bar at the top of the screen, and a home screen with a dock and centered application menu), regardless of screen size
- Increased number of extended notifications and Actionable Notifications for more applications, allowing users to respond to certain notifications within the notification bar and without launching the application directly
- Always-on VPN
- Premium SMS confirmation
- Group Messaging
- Added Bluetooth gamepads and joysticks as supported HID (Human interface device)
- Long-pressing the Wi-Fi and Bluetooth icons in Quick Settings now toggles the on/off state
- New download notifications, which now shows the percentage and estimated time remaining for active application downloads
- New sounds for wireless charging and low battery
- New Gallery application animation allows faster loading
- USB debug whitelist
- Bluetooth low energy support which means support for Bluetooth 4.0
- Bluetooth Audio/Video Remote Control Profile (AVRCP) 1.3 support
- OpenGL ES 3.0 support, allowing for improved game graphics
- Restricted access mode for new user profiles
- Filesystem write performance improvement by running fstrim command while device is idle
- Dial pad auto-complete in the Phone application
- Improvements to Photo Sphere
- Reworked camera UI, previously introduced on Google Play edition phones
- Addition of "App Ops", a fine-grained application permissions control system (hidden by default)
- 4K resolution support
- Many security enhancements, performance enhancements, and bug fixes
- System-level support for geofencing and Wi-Fi scanning APIs
- Background Wi-Fi location still runs even when Wi-Fi is turned off
- Developer logging and analyzing enhancements
- Added support for five more languages
- Improved digital rights management (DRM) APIs
- Right-to-left (RTL) languages now supported
- Clock in the status bar disappears if clock is selected as lockscreen widget
 Kit-Kat (V4.4)
- Refreshed interface with white elements instead of blue
- Ability for applications to trigger translucency in the navigation and status bars
- Ability for applications to use "immersive mode" to keep the navigation and status bars hidden while maintaining user interaction
- Action overflow menu buttons are always visible, even on devices with a hardware "Menu" key, which was officially deprecated by Android 4.0
- Optimizations for performance on devices with lower specifications, including zRAM support and "low RAM" device API
- Wireless printing capability
- NFC host card emulation, enabling a device to replace smart cards
- WebViews now based on Chromium engine (feature parity with Chrome for Android 30)
- Expanded functionality for notification listener services
- Public API for developing and managing text messaging clients
- New framework for UI transitions
- Storage access framework for retrieving content and documents from other sources
- Sensor batching, step detector and counter APIs
- Settings application now makes it possible to select default text messaging and home (launcher) application
- Audio tunneling, audio monitoring and loudness enhancer
- Built-in screen recording feature (primarily for developers, as usage of ADB is required)
- Native infrared blaster API
- Expanded accessibility APIs and system-level closed captioning settings
- New experimental runtime virtual machine, ART (not enabled by default)
- Bluetooth Message Access Profile (MAP) support
- Disabled access to battery statistics by third-party applications
- Settings application no longer uses a multi-pane layout on devices with larger screens
- Wi-Fi and mobile data activity (TX/RX) indicators are moved to quick settings
- Applications' access to secondary storage (memory cards on devices with internal primary storage) is restricted to their designated private directories only, while full access to internal primary storage is still allowed
- Improvements to auto focus, white balance and HDR+ for the Nexus 5 camera
- Better application compatibility for the experimental ART runtime
- Camera application now loads Google+ Photos instead of Gallery when swiping away from the camera view
- Removal of the "App Ops" application permissions control system, introduced in Android 4.3
 Lollipop (V5.0)
- A bold, colorful, and responsive UI design for consistent, intuitive experiences across all your devices.
- Notifications - New ways to control when and how you receive messages - only get interrupted when you want to be.
- A battery saver feature which extends device use by up to 90 mins
- New devices come with encryption automatically turned on to help protect data on lost or stolen devices
- Device Sharing has more flexible sharing with family and friends
- Get to the most frequently used settings with just two swipes down from the top of the screen
- A better internet connection everywhere and more powerful Bluetooth low energy capabilities
- A faster, smoother and more powerful computing experience
- Support for 64 bit devices, like the Nexus 9, brings desktop class CPUs to Android
- Bolder graphics and improved audio, video, and camera capabilities
- Easy access to information and performing tasks - Talk to Google even when the device is in standby.
- Android TV - Support for living room devices
- Enhanced low vision and color blind capabilities
- Now in 68+ languages
- Easy Set-up
- and more - Tap & Pay, Print Preview, revamped interface, device level feedback, Android beam for file sharing, improved keyboard support.
 Marshmallow (6.0)
- make notifications more consistent
- make search functions more intuitive
- improved battery life
- gives users more fine-grained privacy controls
- The most prominent feature is probably Now on Tap, which integrates Google searches and voice interaction into individual apps
- Google Now voice search feature competitive with Siri and Cortana
 Nougat (7.0)
- Split-screen app viewing.
- Future virtual reality
- seamless update support but only for new products.
- Improved and expanded notification actions, including replies.
- Improved quick-access toggles.
- Better controls for data and power.
- New emoji with Unicode 9 support.
 Oreo (8.0)
- Picture in Picture - video can be reduced to a corner of the screen.
- Notification dots - tap for news
- stronger security
- longer battery life
- higher speed - does more locally
- autofills - remember logins
- emojis - redesigned
 Pie (9.0)
- Notch support for displays.
- adaptive battery - to make the battery last longer.
- tools to measure usage
- simpler navigation style that uses just one central button to go home, open recent apps and call up the Google Assistant, plus a back button
 Android Go Edition
Beginning with the Oreo (8.1) version there is a split off for a smaller and lighter Android OS to be used primarily on entry level products.
Android Oreo (Go Edition) has been include as part of the Android Oreo 8.1 rollout, and all Android Oreo devices with 512MB to 1GB of memory will be optimized for Android Go. Google said this will allow them to function properly as smartphones while doubling their available storage space. The rollout includes:
- An operating system designed for better performance, storage and security features on low-end phones.
- A new set of "lighter" Google apps.
- A Google Play store that highlights the apps designed to work best on entry-level devices.
- smaller kernel with less preinstalled apps.
- More storage space available (2X)
- More control over how things are stored.
- smaller apps (50%).
- A new app to help find and remove unwanted content.
- Supports Google Assistant.
- Same security as regular Oreo.
 EBook Readers
The following eBook readers run on Android OS. Note that many Tablets can be used as eBooks reader.
- Tolino Shine
- All Onyx BOOX devices, lots of products.
- Barnes and Noble readers use their own version.
- Icarus readers
- Kobo Vox
- Good eReader 13.3
- Pandigital Novel
Android applications, apps, are sometimes downloadable from the Internet and will have an APK extension. Even eBooks will often have this extension. Most of the time you will need a market app to download the file.
 eBook Reading
Applications specific to eBook reading include (in alphabetical order):
- Aldiko Book Reader
- And Bible
- CBD Reader
- Cool Reader
- Google Books
- Kindle Reading Apps
- Mantano Reader
- nook app
- Soc.io eReader
There are lots more: See eBook software#Android
 eInk devices
- Android apps that work well on eInk devices : http://www.mobileread.com/forums/showthread.php?t=242470
- Onyx eBook readers under the BOOX brandname use Android OS.
 web browsers
- Looking for a Browser that isn't Deeply Flawed : http://www.mobileread.com/forums/showthread.php?t=245110
On newer devices Android Market has be renamed to Google Play. If your unit has Android Market preloaded you are likely to get Most of your apps from there. Google, supplies Google Play, also called an app store to select top of the line fully featured units. However many Tablets and phones do not include the Google Play app since their hardware may not have all of the required options so here is a list of sources of app stores or download sites with some comments:
- A list of free apps is available from http://www.freewarelovers.com/android. They also have some apps for older releases which can be very important if you hardware doesn't have a current version available. These are dependent on user uploads. You download .apk files to move to your device and install. There is no app store or manager application.
- http://www.appslib.com is a site that actually verifies some of the apps and has a large collection. Appslib will only work for certain manufacturers. See the site for the list of partners.
- Google Google Play store has its own set of limitations but it is based on capabilities of the device. Only devices with a GPS and a camera can qualify to install Google Market from the factory, although some hackers have added it to custom built firmware. It cannot be downloaded and installed like an app. Thus it is only for full featured device makers who are working with Google.
- Amazon Appstore for Android also has a apps for its apps manager. It does not work well on an Archos 7 (bug submitted) but will work on many devices. It is very restrictive. Every app you get from them is locked to the actual Amazon market app manager. If you uninstall that app all the previous downloads will no longer run as they always check every time they are started.
- Appolicious App Directory. This is not an app store but is a reference store identifying and reviewing apps. It is a good store to help find apps. It will redirect downloads to Google Market, so it only really works if you have Google Market.
- Samsung apps. The app store is preinstalled on Samsung devices. It is being rolled out country by country. There is a pc download tool as well.
- Soc io Mall - A mall for Android, includes apps, books, music. This is a brand new promising source for downloads.
- Handango has apps for Android and lots of other portable platforms. They have been around for years.
- Slidme.org - A community and market app site. You can direct download apk files for sideloading.
- AndBot - Apps & Games, News & Reviews - The source for all things Android.
- Camangimarket - Download the market app for access or download directly. An account is required.
- GetJar.com - free apps. Pick your device. Download from http://m.getjar.com Not all apps work on Android.
- Panda Apps - Android apps click to download.
- Curvefish - some apps reference Android market but if there is a download link then it is a direct download, shift click it.
- Applbrain - can install from the device itself using this App.
- Tablified - points to android market for downloads but it useful to identify apps that are targeted at Tablets instead of phones.
- Android freeware - another free Android apps directory. No account is required, however a camera and Android code reader is required, and you have to wade through adds.
- Android Open-Source-Apps - open source Android apps directory. No account is required.
There is an emulator available from http://developer.android.com/guide/developing/tools/emulator.html that can be used during development of an App or to run one on your PC.
BlueStacks is another emulator that is intended to run Android apps on your Home PC.
The latest Chrome OS can run Android apps.