Application software (also called "app") is a program which is used to perform a useful function or a set of functions for its user. The term is used especially often for mobile apps - small products which you can download and run on your smartphone. Back in 2014, the number of mobile phones in the world has exceeded the number of all living people, and that trend continues to this day. Because of the enormousness of that market, launching a mobile app is considered critical for any successful startup at some point, if at all possible. This holds true for Bitcoin startups too, so one can find hundreds of different apps of Bitcoin wallets, exchanges, marketplaces, and other services on Apple App Store and Google Play.