An App is a short term used to describe small application that runs on mobile computing devices (phones, tablets). Before the advent of the smartphone, mobile phones mostly did two or three things – mainly, they were just that, phones. If you wanted to take professional photos, surf the web, edit holiday videos watch music videos, films or listen to music you would pick up a laptop, a dvd player or a walkman / ipod. These days, all of these devices can me replicated / replaced by a software app on the phone in your pocket.
Now, the telephone capability is probably one of the least used functions on a mobile phone !