The term app originally comes from the English language and is derived from the word application. In German, this stands for application. In the general understanding, an app often stands for an application that runs on a smartphone or mobile device. It is called a mobile app.
These mobile apps can be downloaded from the Internet. There are often special stores such as the AppStore or the Android Store. They are free or require payment. All apps should bring their users a specific user, such as entertainment through a game or calculation of the alcohol level. That is why they are usually designed for a specific function.