some guy asked the same question earlier.
UV lights are contained usually beneathn the tank, you have a pump which pumps water through the UV enclosure. this kills everything in the water. this is good for a fish only tank. VERY BAD for a reef tank. what makes a reef tank work is the fact that the water is filled full of little mirco-organisms. when you kill these things the reef stops working and will collapse.
VHO lights on the other hand are lighting for you main tank, so your critters can grow and be happy.