It is a filter system. I made mine for my 55 from a 20 gallon tank. There is an overflow box on the tank, which siphons the water down to the sump. Once it hits the sump, it goes down through a vertical pile of bioballs suspended off of the bottom of the tank, then flows through a secondary sponge, ans to the return pump. This then pumps it back to the tank. This is a VERY basic theorey of what a sump does. Try searching the postsa for the word sump, and you should find some good stuff. If not search yashoo or something. Good luck, If you need more details I could tell you no prob, just let me know,
Adam