Xenon gas (HID lights) are utilized in a similar fashion to that in halogen-filled light bulbs. Instead of employing a filament though, a high voltage electrical arc is created between electrodes and a collection of metal halide salts that are pressurized inside the bulb. This reaction creates the powerful illumination of high intensity lighting, up to and including 10k, where it takes on the bluish tone.
In very simple archaic, terms, think of the HID light as a small spark-plug, in a xenon gas filled, sealed environment, but there is no on/off of the spark, it is a constant electrical arc. When they were first designed, the lights would have taken several minutes to power up. Well, this was an unacceptable wait for driving, so they used an automotive electronic ballast to temporarily accelerate the electrical power delivery during warm ups, and for maintaining the lighting arc. Florescent bulbs also use this type of technology, as, there is no filament in a florescent bulb either, it is just pure electricity that illuminates the gas.
So, with the popularity of HIDs, it has spawned a sizable aftermarket in headlight "conversion" kits. It's important to note that the majority of these headlight kits are not approved by the Department of Transportation for road use. It is also recommended before installing any HID lighting in your vehicle, that you check with state transportation laws.