It's just the lighting temperature that produces a spectrum most helpful to corals. When you get bulbs, try to get them at 10000k+, if not 14000k or 20000k. Power compacts, VHO
and metal halides are different types of bulbs in increasing order of brightness. Metal halides are the best for most corals, but by and far the most expensive. Incandescent bulbs or what not aren't fit for coral growth and have a highly yellow spectrum.
Basically, just try to find a decent fixture that offers in some actinic supplementation, along with being at least 2, preferably 3 watts per gallon. You can have fish no problem with less, but you need to make some investment in lights to grow corals unless you have a sunlight amplifying system, which is unpractical for most.
I have a 24g jbj nano with 2 stock 36 watt 50/50 10000k PC
bulbs, which means they are half actinic and half daylight bulbs. This means I have 3 watts per gallon which is more than enough for many soft corals and indeed some stony corals.