APRS was developed by Bob Bruninga, WB4APR, for tracking and digital  communications with mobile GPS equipped stations with two-way radio. In the  12 years since its introduction, it has grown to encompass just about every aspect  of Amateur Radio. 

APRS is a real-time tactical digital communicatons protocol for exchanging information between a large number of stations covering a large (local) area. As a multi-user data network, it is quite different from  conventional packet radio.

APRS is different from regular packet in four ways. First by the  integration of maps and other data displays to organize and display  data, second, by using a one-to-many protocol to update everyone in real  time, third, by using generic digipeating so that prior knowledge of the network is not required, AND FORTH, since 1997, a worldwide transparent  internet backbone, linking everyone worldwide. APRS turns packet radio  into a real-time tactical communications and display system for emergencies  and public service applications (and global communications). Normal  packet radio has only shown usefulness in passing bulk message traffic  (Email) from point to point. It has been difficult to apply conventional  packet to real time events where information has a very short life time and needs to get to everyone.

Although the recent interfaces to the Internet make APRS a global  communications system for live real-time traffic, this is not the primary objective. But like all of our other radios, how we use APRS in an emergency of special event is what drives the design of the APRS protocol. Although APRS is used 99% of the time over great distances, and benign conditions, the protocol is designed to be optimized for short  distance real-time crisis operations.

APRS provides universal connectivity to all stations by avoiding the complexity and limitations of a connected network. It permits  any number of stations to exchange data just like voice users would on a  voice net. Any station that has information to contribute simply sends it, and all stations receive it and log it. Secondly, APRS recognizes  that one of the greatest real-time needs at any special event or emergency  is the tracking of key assets. Where is the Event Leader? Where are the  emergency vehicles? Whats the Weather at various points in the County?  To answer these questions, APRS is a full featured automatic vehicle  location and status reporting system too. It can be  used over any 2-way radio system including HAM, CB, Marine Band, and Cellular Phone.

You don't need to own a GPS receiver to enjoy APRS. All you need is the APRS software and your normal packet TNC. Just determine your latitude and longitude as best you can. Look it up in an atlas, or borrow a friend's GPS receiver just long enough to determine the position of your station. After you feed the information to the software, your TNC will regularly announce your position to anyone else who is monitoring. You can even use APRS to exchange bulletins and enjoy live conversations with others on the network. 

APRS is on 144.39 throughout the North American Continent. Other countries often use other frequencies. Check locally.