A good place to start would be the definition of Geo-Fencing on WhatIs.com.
Beyond that, Geo-fencing in mobile is a big topic. Are you looking for types of geo-fencing applications? I’m with Maponics, for example, and our customers create geofences for marketing programs, sales campaigns, public safety apps, asset/inventory management, health care apps, etc. There are tons of possible applications of geofences. Is this the type of info you were looking for?