From SKYbrary Wiki
A turbocharger is a turbine driven, forced induction device that is utilised in a piston engine to enable it to produce more power.
In a normally aspirated piston engine, intake gases are drawn into the cylinder by the reduced pressure created by the downward stoke of the piston. The mass of the air that is drawn into the cylinder, in part, limits the power production of the engine, A turbocharger recovers waste energy from the engine exhaust stream and uses it to power a turbine which in turn drives a compressor. This compressor draws in ambient air, compresses it and then feeds it into the engine intake resulting in a greater mass of air and, proportionally, a greater amount of fuel entering the cylinders on the intake stroke. This increase in air and fuel results in the turbocharged engine being more powerful and efficient than its naturally aspirated counterpart.