Sorry guys, its a KEY-USB cable.
Its basically a diagnostic tool, you can check the health of all sorts of things in your engine etc. You can look for boost leaks, check MAF readings to see if you're getting the power you should etc.
Check out the vagcom forum on here, or
http://www.ross-tech.com/
You need a bit of software, but you just download it, the cable is the dongle.
Most guys swear by it on here. You can do a healthcheck that dealers will charge £75 plus in a matter of seconds so it pays for itself pretty quickly.
Cheers
Matt