Application

An application program is computers software to help users "apply" specific tasks. On all computers, Linux, Windows and OS X, standalone programs have generally been known as applications (or apps for mobile devices).

In layman's terms, apps are the pieces of software that run on top of an operating system. For instance, a Mac's usefulness would be very limited without any web browsers, file viewers, media editors, games, messaging programs, mail readers, etc., especially without the Finder which lets you manage your files and launch other applications.