In a fuel injection engine, the throttle body is the part of the air intake system that controls the amount of air flowing into the engine, in response to driver input. While the accelerator is often called a "gas pedal", it would be more accurate to describe it as an "air pedal".
The throttle body is usually located between the air filter box and the intake manifold, and usually attached to, or near, the mass airflow sensor.
The largest piece inside the throttle body is the throttle plate, which is a butterfly valve that regulates the airflow.
On many cars, the accelerator pedal motion is communicated through the throttle cable, which activates the throttle linkages, which moves the throttle plate. In cars with electronic throttle control, an electric motor controls the throttle linkages.
When the driver presses on the accelerator pedal, the throttle plate opens up, allowing more air into the intake manifold. The airflow sensor measures this increase in airflow and communicates that to the ECU. The ECU then increases the amount of fuel being sent to the fuel injectors in order to maintain the desired air-fuel ratio.
Throttle bodies also contain valves and adjustments to control the minimum airflow during idle. Many cars have a single throttle body, however more than one may be used, chained together by linkages, to improve throttle response. At the extreme end, cars such as BMW's M line have a separate throttle body for each cylinder.
A throttle body is somewhat analogous to the carburetor in a non-injected engine. Carburetors combine the functionality of the throttle body and the fuel injectors into one, that is, to modulate the amount of air flow, and to combine air and gas together. Cars with throttle body injection (called TBI by General Motors and CFI by Ford) locate the fuel injectors in the throttle body, allowing an engine design to be converted from a carburetor to fuel injection without changing the intake layout.