Continuous data versus defined events
GPS tracking
GPS tracking captures a device's position repeatedly and stores the trail. It produces a continuous stream of coordinates that can be replayed as a route. The question it answers is where has this person been, and it answers it whether or not anything meaningful happened.
Geofencing
A geofence is a virtual boundary drawn around a place, usually a customer location, with rules attached. When a device enters, leaves or stays inside the boundary, an event is recorded. It produces occasional events rather than a stream, and the question it answers is did this visit happen.