Often apps have a very specialize purpose which has led to the phrase that "There is an app for that." An App can also be an eBook when the eBook reading program is built into the book itself. An App, like an Application, must be designed for a particular OS although there may be multiple versions of the app for different OS's. Apps are sometimes written as an alternative web viewer instead of just using a Web Browser on a certain site where they can offer a better screen experience.
Another use for apps are as extensions, also called plug-ins, for Web Browsers that perform specific functions, often unrelated to actual browser use. This is used primarily with Cloud OS's like Chrome OS and Firefox OS. This form of app is often written in Java and may be available on all platforms that support the Browser. They may also be additions to the Safari, Chrome, or Firefox browsers themselves running on a regular OS.
 App Stores
While apps are sometimes available directly from the company that wrote the app, more often apps are available only from an app store. All of the major OS's have app stores specifically for their mobile devices and in some cases the app store has apps for desktop computers as well. (Beginning with version 8 of Windows there are also apps available for desktop computers.) Generally the app store is an app itself that is preloaded with the OS. The device app store may include eBooks, music, videos, games, and hardware as well as apps but these are usually in a different dedicated app.
The advantages of an app store includes:
- Direct access and purchases available from one source already on the device.
- Some expectation of security in that the app store released the app and has verified it. This is increasingly important with virus laden products downloaded direct from web sites.
- The app store app will usually keep track of which apps you own and can restore them if they are deleted and may even be able to offer updates and upgrades.
- Purchasing a paid app is easier with a central source to handle the transaction.
 eBook apps
It is also possible to make an eBook that is really an app. The idea is that you can read the book without using any external application to do it. The executable itself displays the book page by page. Do do this you need to include a program and the book in the same file. The advantage is that you have control over exactly how the book will look. The disadvantage is that you will need a separate program for every platform (OS). The book will also be larger since the program to read it is inside the file. If you are publishing more than one book you can reuse the program with only minor changes to the new book. Generally this will require programming skills to write the program although you may be able to modify some existing source files for your use. You could also hire someone else to write the program. There are also a few programs already out there that can be purchased to make your own self running book. See EXE for more details.