A sump is nothing more than an extra water containment area usually set up underneath an aquarium where you can place equipment, provide the aquarium with various types of water filtration, add top-off water to the tank and perform other maintenance tasks. All this means that you can have rocks, sand, macro algea, skimmer, heater, part fuge, part sump, part wet/dry, just about anything that you can do to it. Lots of combiantions and designs and functions.