App is an abbreviated form of the word "application." An application is a software program that's designed to perform a specific function directly for the user or, in some cases, for another application program.
Different types of apps include:
Web app - stored on a remote server and delivered over the Internet through a browser interface.
native app - has been developed for use on a particular platform or device.
hybrid app - combines elements of both native and Web applications. Hybrid apps are often mentioned in the context of mobile computing.
killer app - a program that intentionally or unintentionally gets you to buy the system the application runs on.
legacy app - has been inherited from languages, platforms, and techniques earlier than current technology.