App is an abbreviation of the word Applications which is a computer software program. This term is often used to for small single purpose software program with a touch screen interface used on a Web Tablet. Other usage includes the Portable Apps.
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 for the app for different OS's. Apps are sometimes written as an alternative 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.
 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.