A turbocharger is a device to force additional air into an internal combustion engine. The additional air increases the engine's power and efficiency to the point where most racing series have separate regulations or classes for turbocharged engines.
Often called "turbos" for short, they work by using the engines own exhaust gasses. Internal combustion engines require a mixture of fuel (usually gasoline) and air in the cylinder for combustion. A spark combusts the mixture, creating a small explosion, forcing the piston down. This is the power stroke, which propels the car. The piston then moves back up in the exhaust stroke, expelling the used gasses into the atmosphere.
In a turbocharged engine, the exhaust gasses pass through a turbine, which is directly linked to a compressor in the air intake. By compressing the incoming fresh air to the engine, each cylinder is able to generate more force in the power stroke, boosting the engines overall power rating, without increasing its size.
The first Formula One car to be powered by a turbocharged engine was the Renault RS01. Designed by André de Cortanze and Jean-Pierre Jabouille, the car had a 1.5 liter V6. Modern F1 cars use turbocharging along with kinetic recovery systems to maximize overall propulsion efficiency.