What is a turbocharger?

A turbocharger, or turbo (colloquialism), from the Greek "τύρβη" (mixing/spinning) is a forced induction device used to allow more power to be produced for an engine of a given size. A turbocharged engine can be more powerful and efficient than a naturally aspirated engine because the turbine forces more intake air, proportionately more fuel, into the combustion chamber than if atmospheric pressure alone is used.

 Turbochargers were originally known as a turbosuperchargers when all forced induction devices were classified as superchargers, nowadays the term "supercharger" is usually applied to only mechanically-driven forced induction devices. The key difference between a turbocharger and a conventional supercharger is that the latter is mechanically driven from the engine, often from a belt connected to the crankshaft, whereas a turbocharger is driven by the engine's exhaust gas turbine. Compared to a mechanically-driven supercharger, turbochargers tend to be more efficient but less responsive. Twincharger refers to an engine which has both a supercharger and a turbocharger.
Turbos are commonly used on truck, car, train and construction equipment engines. Turbos are popularly used with Otto cycle and Diesel cycle internal combustion engines. They have also been found useful in automotive fuel cells.