HIDs (high intensity discharge) work in a similar way to a fluorescent light in that they use a ballast and igniter to ignite a gas in the glass bulb using a very high voltage. The gas creates a much brighter light than any filament halogen bulb can create. HIDs also use less energy to run. HIDs are what you will see the newer STIs using as stock headlights. BMW, Audi, and Mercedes have been using them for quite some time also - the purple looking lights you see.
You will want to note that halogen headlights are not designed for HIDs but HIDs will work in them. HID bulbs are longer but will typically require the headlight to be aimed down a little once they are installed. Our HID kits are plug and play and require no cutting or splicing of OEM wiring when installed into your low beams. HIDs are also not DOT approved when installed in a non-HID designed housing. That said I have never heard of someone being required to remove them due to a violation.
You will see a huge improvement in the light output from your headlights after installing HIDs. They will light up a lot more of what is in front of the vehicle and you will be able see more clearly.