HID's arent just a bulb.. you need an ignitor to start the arc and a ballast to convert the charge to a high enough voltage to activate the ignitor... then come the HID bulbs themselves. you need a wire harness to plug your existing wires into, then as you turn on the switch, it goes thru the rest of the HID system to the bulbs.. depending on the car, you can do this with little to no modifications.. but sometimes, a lot of work has to be done to make it compatible.
i upgraded my car to HID's earlier last year and did a lot of research on it.. it was the first time i ever worked on my car for anything, but i was still able to do it.. so, i have no doubts that some of you who have even a tiny bit of automotive experience could do this as well.. aftermarket kits aren't DOT certified though.