sorry - typing too fast, just had coffee
MH is metal halide - the 175w HQI bulb you mentioned.
PC is indeed power compact
Metal Halides are very bright, but get fairly hot.
Powercompacts are slightly less bright per watt compared to MH, but run cooler.
regular flourescent bulbs are signifigantly less bright per watt then either MH or PC bulbs.
Actinic bulbs are "blue" bulbs that provide two main benefits:
a) they make most MH bulbs look "better" (ie less yellow, especially 6500k and 10,000k bulbs) - balancing the tank color out
b) they induce many corals and other inverts to "fluorescence" or glow with interesting colors that are not seen without the actinic lights.
My reef tank with just my 175w (10,000k) MH light looks completely different than it does with my MH and actinics, or even actinics alone.