Mechanical ventilation systems are those which incorporate a fan or similar device to assist air movement. 

An important subset of this main category is mechanical ventilation with heat recovery (MVHR).  These systems extract heat from exhaust air and transfer it to incoming fresh air, greatly reducing the heat energy which would otherwise be lost from a building. Energy is transferred via a heat exchanger, of which there are several designs on the market. Contraflow heat exchangers are amongst the most efficient.