APK is a format specifically for Android devices. It uses a .apk extension and can be downloaded via Android software sources.
 The format
This file format is a variant of the JAR format and is used for the distribution and installation of bundled components onto the Android mobile device platform.
An APK file is a ZIP file, with a particular internal file layout that allows it to be run in place, without unpacking. This format usually consists of 2 folders and several files. This includes:
- META-INF (folder)
- res (folder)
- classes.dex - a compiled Dalvik Executable. (The App)
Android Debug Bridge (adb) is a versatile command line tool that lets you communicate with a device. The adb command facilitates a variety of device actions, such as installing and debugging apps, and it provides access to a Unix like shell. ADB is part of Android Studio.
 Android Studio
Android Studio is the official IDE, Integrated Development Environment, for Android development, and includes everything you need to build Android apps. It includes: SDK tools and environment and also emulators for Windows and MacOS X,