Automatic Packet Reporting System
What is APRS?
The Automatic Position Reporting System, allows mobile users to report their current position over radio. This enables the position to be automatically displayed and updated on computers or special radios running APRS mapping applications.
APRS is fundamentally a real-time tactical digital communications protocol for exchanging information between a large number of stations covering a large (local) area. With various implementations and additions APRS has expanded to a world wide network, though it's fundamental objective still remain the same.
The Australian APRS web site has lots of information about APRS.