Powerhead vs Circulation Flow?

I guess I never really paid any attention to this when buying my toys for the tank. First off my tank has been set up since May of 2013. It isn't a reef tank in any way shape or form. I have about 20 pound of Live Rock, crushed coral gravel for the base, one power head, a portion skimmer (SeaClone 100) which I would love to get rid of because of how slow I believe it takes to fill the cup with Skim-Mate and a Canister Filter (Penn Plax Cascade Canister Filter) which is not the best but also not the worst either.



So back to my question.... what is the difference between using something like the Marineland Powerhead as a powerhead or as a circulation (???) device? I guess what I'm asking it what is the difference between the use of the product as either a Powerhead or as a Circulation device? Seems to me that this is the same thing isn't it? I mean being used either way moves water around the tank right? I'd appreciate it if someone could help me understand this a little better.





Marineland 1200:

Powerhead Flow Rate: 295 GPH

Circulation Flow Rate: 1300 GPH

or as a pump...