Apps (abbr. for application, synonym for program) is the name for executable software, which provides additional functionality or improvements to the user, which goes beyond the scope (or are not includes) of the operating system. Generally, the term app can be used as a synonym for multiple different applications, which may be used in many different areas and use cases, however, the term is established in the area of mobile operating systems as software that provides functionality on these devices (e.g. for Android, iOS, Windows Phone or Symbian).
The term app can be used for every operating system which allows the installation of additional software. To better distinguish apps, which are specifically created for a mobile usage (e.g. on a smartphone), such apps can be called mobile apps. Usually, such apps can be purchased or installed via a so-called app store. Such app stores are mostly pre-installed on the devices, e.g. the Google Play Store or F-Droid.
Mobile apps are developed to provide value to the user and have a special focus on an easy usage. During the development of a mobile app, a special focus is on the user interface and how the app is supposed to be used, as a mobile devices provides only limited space on the screen. If a developer wants to reach users of multiple different platforms, there are basically two approaches: a native app or a mobile web-app. To support multiple platforms with a native app, the developer has to develop an app for each of the platforms they want to support. Mobile web apps usually just require a web browser, however, they mostly require an internet connection at any time.