For what its worth, here's the nerdy technical explanation.

HID, or High Intensity Discharge, usually refers to headlights that use bulbs that are filled with a mixture of xenon gas, mercury, and some salts, and no metal filament. The light comes from an arc of electricity that excites the mixture and glows brightly. This is different from halogen bulbs, which have a metal filament that glows brightly when electricity passes through it.

One frequently asked question is: will putting xenon gas bulbs into the fixture in my headlight that had halogens work the way I want it to?
Answer: Nope, and it could be dangerous. Xenon bulbs require more current when they are first turned on than they do at normal operating temperature. Your headlights designed for halogen bulbs don't have the proper hardware (called a 'ballast') to do this. Additionally, the xenon bulbs produce a different pattern of light than halogen fixtures are designed for, so you could end up with wierd hotspots and a very funny spread of light.

That purple/blue you see from the Maxima's headlights isn't actually produced at the bulb. That effect comes from the way the light is diffuse (read: unfocused) at the edges of the beam, and its caused by a projector lamp lens (think of third gen Integras, all new BMWs, Acura TSX and TL).

In my opinion, the really nice thing about xenon HID lighting is that it produces a much fuller spectrum of light than do halogen bulbs. In my experience, its just easier to drive for long periods at night with the better lighting.