APRS stands for Automatic Position Reporting System. It has been a long work in progress. Throughout the 80s and 90s there was a digital mode called packet. Ten years before the internet started to get popular, hams were connecting computers over radio waves. With the advent and popularity of the internet, packet radio has fallen out. While there are still digital modes widely used on HF because the frequencies travel farther, VHF does not propogate as far and is rarely used for classic packet as it once was. Instead, the packet language is used in the APRS network.
The RF Portion
The most commonly used APRS frequency is 144.390. It uses FM modulated packet radio to communicate. If one puts a scanner or other FM receiver on 144.390 he will probably hear packet bursts being intiated between computers and other devices.
The computer portion
APRS links a GPS receiver and other digital equipment to a ham radio and shares the information with anyone listening. A receiving station has a computer program that contains a map. As stations are heard their positions are placed on the map as well as any information they are sharing.
This is a standard APRS map with all of the stations on it. It is in the LA area. As you can see the mobile stations have a red line next to them. This indicates the direction they are currently traveling. The faster they are going, the longer the line.
If one double-clicks on a callsign it gives a detailed view:
It shows the GPS coordinates, the time the station was last heard, its course, and its height above sea level. It also shows the speed.
Other stations have a WX icon which means they are sharing weather data. Here is a weather station:
As one can see, it contains the GPS coordinates of the weather station and the conditions at the location. During huricane Catrina APRS stations were used to track wind conditions.
One can also bring up a list of stations and a lot of information about them: