An exhaust gas driven turbine, that is directedly connected to a centrifugal compressor unit. It is used to take some of the waste energy from the hot exhaust, and make it do some useful work. The compressed air output from the compressor side needs to be cooled first (there is some dispute over the name for ths cooler, technically it is an After-cooler, although many people call it an Inter-cooler). The cooled compressed gas is then pushed into the Inlet manifold, where it is ready for use in the cylinders to burn the fuel. The increased mass of oxygen in a fixed volume of compressed air allows more fuel to be burned, or for a much cleaner, cooler burn of the same amount of fuel.
Turbo-chargers (sometimes called Turbos), are basically turbines, and as long as the oil supply is clean and good, and the exhaust gas doesn't get too hot they are very reliable. Cleaning of both sides of the turbo is required frequently to remove an unwanted build-up of soot and dust. As the Turbo spins very fast (10,000 to 100,000 rpm depending on size, weight and design), care must be taken in maintaining them. A turbo 'letting go' and shedding its blades is not a pretty sight, as well as being expensive.