Power Compact (PC) lights are basically fluorescent lights bent in half. Power Compact bulbs require a ballast to drive them and are available in a number of Daylight, Actinic Blue and 50/50 spectrums. The wattages available range from 9 watt to 96 watt. PC's come in "straight pin" (4 pins arranged in a straight line) and "square pin" (4 pins arranged in a square). As science marches on, medium base screw-in (like the incandescent bulbs in your home) PC bulbs in Daylight, 50/50 and Actinic Blue have been created. SmartPaq also now make a 17" Quad Daylight and 50/50 in a Square Pin configuration.
Screw-in Mini Compact (like the incandescent bulbs in your home) are made by Coralife and others. Mini Compact bulbs have their own built-in ballast. These relatively inexpensive, cool running, long life bulbs are available in Daylight, Actinic and 50/50 spectrums. For the DIYers, these bulbs are also available in a wide variety of wattages and spectrums at home improvement retailers.
Mini reef tank lighting has been a challenge in the past. The invent of the smaller (down to 12") Power Compact fixtures has filled that gap with a number of high output Mini PC's are now available at reasonable prices.