I dont think anyone has actually explained what HID is, so I will do it to the best of my ability. If i miss something or get it wrong someone correct me. Basically, HID stands for High Intensity Discharge. Instead of using a filament like normal bulbs, it does something else (im actually not sure what.) It uses a different gas than normal halogen or xenon bulbs. Like some other people have said, the K rating of the HID dictates what color light it will project.
i hope that help somewhat