HID = High Intensity Discharge (lamp).
A lamp assembly with a ballast and electronic "gimmickery" that provides for extremely (energy) efficient light. As you know in a halogen bulb, or in a regular incandescent bulb, most energy used just goes away as heat, and is not "converted" into (visible) light.
Blueish/whitish halogen bulbs are very popular at the moment since many believes that they get "HIDs" at a steal.
Not going into it here, but most of these bulbs are actually less efficient (if they have blue tinted glass instead of a normally clear glass on the actual bulb) than regular halogens. Some people like the "blueish" look though. Me, I prefer the better visiblity of "regular" halogens.
All that said, you can have the best bulb, but if you do not have a reflector that is properly matched to it, nothing's gained...
Then on the other hand, DOT rules provides for some strange regulations that just results in ****pier ligh output from halogen light assemblies compared to same vehicle in i.e. in Europe. But, that is a whole other thread.
Have you gotten your tail light yet?