Automatic Packet Reporting System (APRS) is a digital system that uses VHF radio to send small packets of data.
These transmit information and messages over the amateur radio network.
The most common use for these small packets of data is to report the GPS co-ordinates and information.
As a result, this information can then be decoded and mapped to show the position of the transmitting station.
The use of the internet has spawned a number of websites displaying the data in real time, aprs.fi possibly being one of the best.