Perhaps the most important thing to remember when installing HID bulbs in automotive reflectors designed for halogen bulbs is:
DO NOT DO IT!!!!!:thumbsdow It's illegal and unsafe.
Headlamps are very carefully designed to reproduce a beam pattern that puts the majority of light down on the road in front of you, and farther down the road towards the right. They are designed to keep the light out of the eyes of drivers in front of you, either in your lane, or in oncoming lanes.
When you put "HID Upgrade" bulbs in your headlamps, it screws up the beam pattern because of the different shape of the light source. Aside from the fact that they'll be putting out >3000lm when they were probably designed for 800-1200, they WILL reflect light in to the eyes of other drivers. I have to put up with it every night, and it's one of the most annoying things that someone can do to their car.
A $100-300 kit will not make your car's headlamps work like OEM headlights made by Bosch or Hella, as used on many European cars (VW, Audi, BMW, Porsche, Range Rover). Those use precision projector optics to shape the light in to a safe beam pattern, and are legally required to adjust themselves when the car is loaded heavily or drives over a bump to keep the light on the road where it's supposed to be.
Now, if you want to learn about HID lighting in general, rather than how it applies to dazzling drivers at night, you should look around this forum. There's plenty of information about how they work, who makes them, how to use them in spotlight mods, and more.
PS: Perhaps this thread should be moved to the Automotive forum.