Type of the bulb.  Common types include halogen, xenon, and LED.

 

Halogen - The most common type of automotive bulb, used for headlamps and foglights.

 

Xenon - Commonly referred to as High Intensity Discharge, or HID.  Xenon bulbs use a gas arc rather than a filament, and produce much higher levels of light while consuming less wattage.

 

LED - Solid state LED lights feature low power consumption, low heat generation, and HID-like color outputs.