High-Intensity Discharge (HID) are quite a bit different from traditional halogen bulbs. Rather than using a standard filament, HID lights work like fluorescent bulbs. Electricity is passed through a gas to create an intense, bluish light.
HID lights are normally much brighter than halogens, which provides a greater field of vision for the driver. They also provide a more distinct light cutoff, which, when the lights are angled properly, make them more comfortable for oncoming traffic.