It was installed with a software update a while back - the one that introduced iMessage I think.
When you sync it a 'messages' icon should be added to your iPad which allows you to message like a phone does. If you have a wifi iPad it will only work when you are on a wifi network, obviously.
It attached to your email address that is connected to the iPad I think. Or at least it used to - or maybe that is just for wifi iPads and 3G ones attach to the 'number' of the sim.