Why APRS stands for Automatic Packet Reporting System. Stations beacon short packets containing GPS position, status, weather data or brief messages, and receiving software plots those positions on a map in real time. That makes it a tactical tool for public service events and emergencies, where net control can see where mobile stations actually are. The usual VHF channel for APRS in North America is 144.39 MHz.
Watch out Voice over internet between repeaters describes linking systems like EchoLink or IRLP, not APRS, and PACTOR is a separate HF digital protocol used mainly for message forwarding.
APRS = position packets on a map. Think "Automatic Position Reporting" on 144.39.