VHO is a "regular" sized bulb that can have tons more power going through it, so the result is it puts out more light as well.
You don't mention them, but there are also HO (High Output), these are basically the product between NO and VHO, more power than NO, less than VHO. They aren't used much however since it doesn't cost much more to just get the more effective VHO.
PC is a really really long bulb that has been wrapped around and around so that it fits in a smaller space. So that way, you can have the power of, say, a 72" bulb in the space of 30."
VHO and PC are very comparable in effectiveness.
Oh, if you are a DIY, don't fall for buying screw in house bulbs that are little PC's. These are not appropriate for aquarium use.