HID High Intensity Discharge, also know as Xenon or Bi-Xenon (which is sort of a lie cause the high beams are still Halogen. Don't be fooled into thinking a bulb that says Xenon gas filled makes them HIDs (like in most luxury cars)
HIDs are filled with Xenon Gas but unlike typical halogen bulbs that run a filament like a typical light bulb, HIDs create an arc in the gas enclosure. they also run at 35 watts where as a typical Halogen uses around 55w. so they are more efficient as far as power drawn.
There are 3 components to lighting:
1 Lumens- how much light is put out
2 Color tempature- the color of the light which causes some confusion:
Halogens usually put out about 3200K (kelvin) which has a yellowish tinge where as most OEM HIDs put out 4300-4800k which is a white light closer to the color of the sun. so more lumens the better right? nope HID kits can go up to 12000k but they don't make any more light in fact all they do is change the color of the light 4300-5300K is the money spot where you have the maximum efficiency for the human eye.
Q: but why do the Audis or BMW look purple from some angles does that mean they are at 8000-10000K?
A: nope they are projector beam so depending on the offset you are looking at them they tend to have prismatic effect.
Q: but what about silverstars or PIAAs they look whiter am I geting as much light as the HIDs
A: nope part of the whiter light you are seeing is a blue coating on top of the bulb. Which although provides a better contrast these coatings actually rob light. which will explain why these bulbs suck butt in the rain on poorly lit roads
3 Absorbtion- how much light sticks to the road versus glaring off of it. This is where HID really beats the pants off of halogens... LED (light emitting diode) technology is even more efficient than HIDs so be on the lookout it will be rolling into dealer showrooms in the next few years.
I've been doing some light reading into the bulbs and peoples experiences and although Silverstars are the reighning champ as far as halogen light output (in dry conditions) they have a tendency of burning out (thus me having to change a bulb after a mere 15k after installing them). The PIAAs are more reliable, but either pale in comparison to an OEM HID setup (like anything you will find in the Infiniti lineup)