A power head is basically a small impeller-driven pump that you can use for various applications in an aquarium, the most used of which is for a UG filter (as opposed to an air pump. They are frequently used to increase the surface agitation and general water flow in saltwater aquariums, as well as in do-it-yourself wet/dry filters, tiny water garden waterfalls, as the source of water flow for a sponge filter, and just about anything else you can think of. Reverse-flow powerheads make for a better UG filter, in some people's opinions.