Many stations have automatic weather stations set up to transmit temperature, wind speed and direction and rainfall data every hour. This allows receiving stations (including the National Weather Service) to collect and display weather data on a map in real-time. During dangerous weather this system can free the Skywarn telephone and voice radio networks for more significant severe weather reporting.
For special events, APRS allows mobile stations (some small enough to be carried by a runner or cyclist) to transmit their position periodically. A relay network conveys this information for graphic display. Again, this can free voice networks logistic communications.
The emergency benefits of APRS was demonstrated during a recent snowstorm in Pennsylvania. An amateur came across (and was almost involved in) a serious motor vehicle accident on the Pennsylvania Turnpike. As a result of the accident, two people were trapped in an overturned vehicle with apparently serious injuries. He was unable to reach 9-1-1 by cellular phone, and did not receive an answer to his "break, break, break, emergency traffic" call on the local repeater.
In a last ditch effort to notify someone of the emergency, he switched his APRS rig into "emergency" mode. This set a flag that triggered all of the stations that heard it to alarm and center their maps on the transmitted position. At least three different amateurs in various areas heard the call and notified Pennsylvania State Police, who responded immediately.
There are two basic classes of APRS stations: telemetry transmitters and those that have some form of user interface.
Telemetry stations consist of a TNC (Terminal Node Controller), a radio, and some form of input device. Two of the most common are "trackers", which use a GPS receiver to transmit the stations position on a regular basis, and stand-alone weather stations that use a Peet Brothers, or equivalent, weather station to monitor temperature, wind, rain, humidity, etc. and transmit it.
Other stations add some form of device to interact with the user. Popular devices are DOS, Windows, and Macintosh personal computers, 3-Com Palm palmtop computers, Linux boxes. The most recently advance is the Kenwood TH-D7A data radio, which provides a radio, TNC and user interface in an HT. Add a GPS and you have a complete APRS station that fits in a cyclist's back pockets.