Naturally aspirated engines rely on atmospheric pressure at around 1013 millibars to force air into the cylinders during the suction strokes of the pistons. Restrictions caused by inlet valves and manifold passages during natural aspiration mean that a full charge of air is never achieved.
Turbochargers boost this pressure to make up the deficiency. A turbocharger is a turbine driven by the exhaust gases which in turn drives a fan (see diagram). This forces air to the engine at a pressure higher than atmospheric, which means that even higher pressure can be generated during the power stroke. This results in higher output torque and increased engine output power.
As the exhaust runs at a high temperature, the materials used in the construction of the turbocharger need to be of a very high quality. Adequate lubrication of the bearings is essential and only high quality engine oil is normally specified for turbocharged engines.
Care is required when stopping an engine which has a turbocharger. Run the engine at tickover speed for a while before stopping. This enables the turbocharger to stop before the engine stops, avoiding starving the turbocharger bearings of lubricating oil.