bulbs, but they do not have a filiment inside, they use less
power then standard bulbs, and they'll cost ya $250+ for crappy ones. You would know if you bought them, they come with their own transformer (ballast) for each bulb thats approx 2x3x1 inches that you have to mount somwhere. anything that you just swap in for stock is not hid's.
For hid bulbs the color is determined by the temprature of the kit you buy. 6000k would be more yellow, 8000K blue, and 10000k would be very purple. the higher the temprature theres more color output but less light output.
they are designed for projector housings, but not cheap visual upgrade ones. they'll melt... real projector housings have horizon cut off lines. a properly installed hid system when shined against a wall will show a straight line of light and dark with no fading. 90% of the color you see will be right at that line. if you put hid bulbs in stock headlight housings you'll be shining light everywhere, it'll look stupid, probably be illegal, you wont get the color shift and you'll probably piss a lot of people off. it'll be like driving with with your highbeams on all the time x 10
This is an improper HID setup, this is what it looks like if you just swap a civic's bulb with a H4 HID, on the left is the HID throwing light everywhere, very illegal, looks like sh*t, the right is the stock bulb
This is a proper HID setup running with 8000k bulbs. The horizon cut off line is perfect. It looks sweet. This is legal and wont piss people off.