Turbochargers are used for a wide variety of things; from race cars to large diesel engines. Generally speaking, turbochargers are used in circuit racing and road courses rather than drag racing. This is because the power is needed at higher RPMs and low-end torque is not as important.
Above: Cars such as the Audi RS 6 (pictured above) come stock with turbos. The RS 6 features a biturbo engine making 450 horsepower. This is a huge jump over the horsepower of the non-turbocharged engine, which is a mere 310 hp.