A mobile app i.e. a mobile application is an application developed for mobile phones and smartphones. In simple terms, a mobile app is a software programme for ones phone. These can be as simple as a basic flashlight application or a dictionary or something similar to a Google Mapsnavigation app.
There are mobile apps developed for entertainment like the Facebook and Twitter app (social media networks) or the Angry Birds mobile app (games). Mobile applications are basically a lighter version of computer applications meant for mobile devices/tablet devices, which cater to infotainment and entertainment.