Wikipedia defines geo fence as a virtual perimeter for a real-world geographic area.
A geo-fence could be dynamically generated - as in a radius around a store or point location. Or a geo-fence can be a predefined set of boundaries, like school attendance zones or neighborhood boundaries. Custom-digitized geofences are also in use.
When the location-aware device of a location-based service (LBS) user enters or exits a geo-fence, the device receives a generated notification. This notification might contain information about the location of the device. The geofence notice might be sent to a mobile telephone or an email account.
A great example of Geo fencing application - Footprint feed