APRS is a system that allow you to track an amateur radio station during his mobile operations. You can also send and receive short messages using APRS. If you are at home you only need a PC, a TNC and a radio to run APRS, but if you are mobile you also need a GPS unit and a special radio with a display and a TNC (often built in the radio). Commercial radios are available with the TNC and a large display to run APRS without problems. Really often APRS is used to exchange messages between fixed stations too. Technically APRS is a system that work sending and receiving a digital signal. Using a radio the signal is converted as audio, and is well known as Packet Radio. The speed of this signal is 1200 Baud. The frequencies used for APRS are 144.800 MHz for Europe and 144.390 MHz for USA. Some trial is done using HF bands and connections with internet. With a radio every while is sent a signal (beacon) that decoded is a string of information such as latitude and longitude (to allow the system to display an icon in the right spot of the map), the kind of icon and other short info (i.e. operator name). If you play with your PC mouse and the appropriate software you can discover that a lot of option are available, including sending messages. APRS doesn't need a connection to run, but will just use the frame UI of packet (AX25). The ack of a sent message will be sent back as UI frame too. Using the right setup a station could be used as a digital repeater and using the right command in the sent string you'll have the right path to reach your desired distance and station. I use UI-View as APRS PC program and on the left menu you can find the basic needs and some maps.