More commonly known as a "sump," an aquarium overflow is a separate tank or water receptacle attached to a main aquarium. The separate tank holds all of the filtration and other support mechanisms for the tank, including a canister filter, heater, degasser, protein skimmer and UV sterilizer.
Sumps are often used in saltwater reef tanks and other tanks that require high stability, although large freshwater aquariums may use sumps as well. A large sump can effectively double the water capacity of a tank, diluting toxic by-products of fishkeeping, such as ammonia and nitrite. It also increases the water quality significantly.