OBD II - On-board-diagnostics (Version 2)!

It's a common interface between your engine management system and the outside world!

It allows you to tap into the brain of the car and monitor sensors, such as speed controllers, injectors, exhaust gasses, etc, etc, basically most things that the engine management system can monitor!

Also, in some cars, you can back modify the parameter tables of your enging management and effectivly "tweak" your performance! For example altering the amount of air/fuel at given revs!

There is lots of stuff about it on the web, try doing a search for OBD in one of the popular search engines,

Mark.