The basic science behind it is that water is passed by a light that emits UV radiation. This radiation kills whats living in the water. What and how much is killed all depends on how strong the lamp is, and how fast the water moves. It's meant to kill algae, parasites, and bacteria. Basically anything living.
As for their track record in an aquarium? I have no clue. I'll let the people who have one speak to that.
I have asked the same question and gotten mixed responses. Some say they are great, some say they do nothing, some say they kill too much.