APRS (Automatic Packet Reporting System) sends short data packets with positions, weather, messages and status. Stations are relayed by digipeaters and passed to the internet by iGates, where you can see them on maps such as aprs.fi.
1The frequencies
The WIA band plan sets 145.175 MHz as the national AX.25 APRS frequency. 145.200 MHz is national AX.25 WICEN APRS, and the 70cm APRS channel is 439.100 MHz.
For reference, the band plan also lists 52.050 MHz for APRS on 6m, and HF 300-baud APRS on 7046.9 kHz and 10147.6 kHz (USB dial).
2Choosing a path
The path tells digipeaters how far to relay your packet. The VK APRS Network recommends:
Urban areas: WIDE1-1,WIDE2-1
Rural, low-activity areas: WIDE1-1,WIDE2-2
Mobiles generally use WIDE1-1,WIDE2-1 nationally, while fixed stations often use WIDE2-1. Fill-in digipeaters respond to WIDE1-1, which replaced RELAY. Don't use RELAY, WIDE, TRACE or TRACEn-n, which are obsolete.
These are conventions, not law. There's no ACMA or WIA rule on APRS paths; they come from the volunteer network, and state coordinators may differ. Check your state's APRS page or coordinator for local advice, especially for fixed stations.
3Good habits
Keep paths short and beacon no more often than you need. Every packet you send is relayed by shared, volunteer-run infrastructure.