HID means High-Intensity Discharge lamps. They provide about twice the brightness of halogen headlights but use less power and can last longer.

From the Associated Press:
These lights are good for the driver, BAD for everyone oncoming!!! A lighting specialist at the National Highway Traffic Safety Administration said some people are more sensitive to light that has a blue cast. But he said the lights meet the same intensity and beam shape standards in place since 1978, the last time the government made a major revision of its standards.

Halogen lamps on most of today's cars generate light by heating a tungsten filament inside a halogen capsule. The halogen gas helps prevent blackening of the capsule as the filament slowly burns out.
High-intensity discharge lamps operate more like vapor-filled streetlights. They don't have a filament, but create light by zapping an arc between two electrodes. That arc excites a different kind of gas, usually xenon, which in turn ignites metallic salts.

Rob Levinson '94 M5 3.8 Touring wiht HIDs in US ellipsoid headlights

Procedure

Some Bmw E34 owners spent $400 to change there bulbs for HID ones.

If you want to be part of this elite, the whole procedure is available in the UUC Motorwerks page using Lincoln Mark VIII parts (it is a PDF file).
The only way to do this mod correctly is with US ellipsoids. Euro ellipsoids will not work (metal housing cannot be modified correctly) and US non-ellipsoid gives a wide, specular pattern with terrible throw (distance of beam projection).

Send me an e-mail if you upgrade your lights with HIDs to create a custom procedure.

Thanks Rob Levinson