The kits are bulbs and ballasts, with associated wires. The bulbs mount just like a standard bulb and they come in the same sizes (H7, H4, etc.). Most times the hardest part is finding a place to mount the ballasts, but they're not that big and the good kits come with mounting "holsters" like in the pic below. I just picked this pic because it's clear - never heard of this business before. If you decide to go this route I would recommend Against the cheap eBay kits, pay the extra money and get a quality kit and you won't be sorry.
Just as an FYI - in comparison with a standard halogen bulb, HID bulbs last, on average, five times longer, use much less energy (35w vs. 55w), put out three times the light, and run much cooler.
Like I said, I absolutely love ours and will never go back to halogen again, of any type. :thumbsup:
(IMG:
http://www.popnow.cn/img/popnowhidbig3.jpg)