There are several different light possibilities, in order of approximate cost.
1.) Regular t-12 florescent. You could keep some extremely low light corals with these, but really nothing but mushrooms.
2.) Power Compact--as stated above, creates a lot of heat. You could get 130 watts, which is pretty intense, but the spectrum isn't that great. These are usually a part of a fixture premade and ready to be mounted above your tank. Would be fine for all softies.
3.) VHO t-12. The t-12 refers to the diameter of the fluorescent bulb. These are high wattage bulbs, driven by a ballast that is usually mounted in the canopy or stand. I think they make 36" ones, but am not sure about 24" VHO.
4.) Metal Halide. These run very hot, and very bright. They are good for keeping all types of corals. You could probably run a 175w single bulb for your size tank.
5.) T-5. These bulbs are much thinner, and as stated by Matt, have a lower wattage, (lower power consumption) but still transfer the same light intensity to the tank as MH. They are a larger initial investment (especially if you get one with the individual reflectors) but would work just as well, and with less heat transfer as well.
When you hear about different types of bulbs, there are basically 2 types. Daylight bulbs and Actinic. Actinic bulbs are blue in color, and make the coral's colors fluoresce more. Daylight bulbs are yellow, white, or bluish white in color, depending on their spectrum. They make everything from 6500 to 20,000 k bulbs for various types (not all of them come with all of the above options...none with regular t-12 lighting.) The higher the number, the more blue the lights get. Usually, Metal Halides are the ones that are really using k (kelvin) ratings, but some PC bulbs do, and even regular expensive fluorescents.
