Here's a good starting point: http://aprs.org/
I won't pretend to be a real expert, but I've used APRS a bit, and look forward to doing more soon.
Some of it's high points -
- local tracking of ham radio resources by GPS coordinates. This is not limited to mobiles. Beacons can be set up to display the location of anything.
This is very handy for public service events - you can give APRS users visual directions.
- global callsign to callsign text messaging via the APRS Internet gateways and/or APRS satellites
- email capabilities
EchoLink, IRLP, Allstar and DSTAR linking - adding interest to repeaters worldwide 24X7