Xenon technology provides a white and brilliant light, with a precise beam for better visibility and greater safety. Xenon lamps provide in general a longer lifetime than halogen lamps.

What is xenon? It is one of the filling gases that gave the lamp its name.
To start the arc, you need a starter which ignites a flash between the electrodes (cathode and anode). 

The arc is a self-sustaining gas discharge between the electrodes.
A ballast converts the low voltage of the car battery to the operating voltage of the xenon HID lamp (HID = High Intensity Discharge).