Refer to figure.
A turbocharger’s main task is to maintain power with increasing altitude as air density is decreasing, therefore making the power available less affected by altitude.

The typical compressor usually fitted in a piston engine’s turbocharger is a centrifugal compressor. The turbocharger is an externally driven supercharger that main advantage over the simpler supercharger is the usage of energy that would otherwise be lost to drive its impeller. That energy is nothing else than exhaust gases. The turbocharger consists of two exhaust gas inlets that receive and direct the exhaust gases towards the turbine wheel. The exhaust gases mechanically spin the turbine before leaving the system through an exhaust gas outlet.

The turbine is connected through a solid common shaft to a compressor, and the rotating energy gained from the exhaust gases circulation will serve to drive the compressor. At the compressor’s end, there’s also an inlet that will receive and direct ambient air to the compressor’s impeller eye. The air will then leave the impeller almost at a tangent to the periphery and trade velocity for an increase in pressure and temperature. The boost air under pressure obtained from this process will then be ducted to mix with the fuel and follow its path towards combustion. The pressure of the compressed air (boost) can be monitored only on the cockpit MAP gauge.