A sump is a glass box, thats it, and some times we put macro algae or rock or sand or all of them in it to aid filtration, the water usually drops from the main tank to the glass box (sump) via gravity to one end, then the water is sucked through the media to the other end and pumped back to the tank, usually its all under the water level, which is unlike a wet/dry which has water dripping onto media exposed to the air.