What are Superchargers?

On this diagram of a Jaguar 4.2L supercharged V8 engine the supercharger and it's pulley have been highlighted in orange

A supercharger (sometimes referred to as a blower) is a compressor used to pump fresh air into the engine cylinders. Because more air, therefore oxygen, is forced into the cylinders this allows the engine to burn more fuel accordingly. The increased amounts of fuel and air in the cylinders cause a larger explosion in the cylinder when the spark plug is fired, subsequently more power is produced A supercharger is powered by a belt, or chain, connected to the engine's crankshaft, just like many of the other ancillaries like the alternator or air-conditioning pump. In the process of powering a supercharger as much as a third of the total crankshaft power of the engine can be used, and in many applications superchargers are less efficient than the closely related turbocharger. However, because a turbocharger is powered by exhaust gases they suffer from 'turbo lag', this is because the pressure of the exhaust gas needs time to build up before spinning the turbine. Superchargers do not suffer from this problem, and in vehicles where power is more important than any other factor (like fuel economy) superchargers are extremely common.