The purpose of putting an amplifier in your car is to increase output. A 2 channel amplifier is meant for a pair of speakers or a woofer where as 4 channel amp can be attached with 2 pair of speakers or 1 pair of speakers and 1 woofer. The 3rd kind is mono amplifiers which are only meant for woofers , bass tubes or bass boxes. The only purpose of mono amp is to increase bass effect in your car.