So, now you're wondering how its done? I know I was. A nice technical details page provides the beginning information about the AIS (Automatic Identification System), which provides ships with pre-collision-detection and identification information.
The receivers ouput NMEA-standard sentences (just like those you get in NMEA mode from a GPS) that provides the information about the ships in question. The information is delivered over two channels capable of handling 2,250 ships each without collision (not yet sure which collision detection mechanisms are used) and is updated dependent upon how fast each vessel is going. Pretty cool.
Unfortunately, NMEA documentation is like international standards from Europe, hideously expensive. A full set of docs for NMEA 0183 will run you nearly USD$300.