Head Lights

I ordered a set of projector headlights with HID's for my car, I'm just asking because all the HID sets I have seen on other cars only have 2 wires coming from there harness and plugging into the HID kit. The kit I ordered said it was for my car but I was just wondering what all 3 wires are for?? I'm sure one is ground and the other is power, but what is the other?