They provide brighter light than halogens while using less energy. They're 3 to 5 times more efficient. They're also supposed to produce a whiter light, closer to daylight and last 3 to 5 times longer.

To get more technical...
HID lights replace the filament of the light bulb with a capsule of gas. The light's emitted from an arc discharge between two electrodes that are closely spaced and hermetically(airtight) sealed inside a small quartz glass tubular envelope capsule. This causes various metallic additives to vaporize, releasing large amounts of light.

:lmfao: @ "tubular envelope capsule"

edit: You beat me to the punch das schmoo, I took too long typing it up.