A circulation pump is kind of like a fan for the water. (The impeller looks like the propeller for a boat instead of being kind of flat.) They typically have a pretty open structure with openings on the side and the end. This is as opposed to a powerhead which has a defined intake and separate output. Typical brands of circulation pumps include Tunze, Korallia, Aqueon, Vortech and Maxi Jet Pro. I use Maxi Jet Pros because of their relatively low cost compared to the other brands. They are also covertable from powerheads to circulation pumps. (The same pump is rated at 160 gph
as a powerhead but jumps to 750 gph
as a circulation pump. It's a differnt impeller and cover, but same motor.) Korallia, Tunze, and Vortech brand can be turned into wavemaker pumps with the use of a controller that times the pumps with each other for different effects. Most other pumps knock at startup shortening their lifespan because of repeated started and stopping which they're not really designed for. I think the Vortechs (and probably some other brands as well) can be run at various speeds for different effects as well.