An electric fuel pump is used on engines with fuel injection to pump fuel from the gasoline tank to the injectors. The fuel pump must deliver the gas under high pressure (typically 30 to 85 psi depending on the application) so the injectors can spray the fuel into the cylinders. Fuel pressure must be within specifications for the engine to run correctly. Too little pressure can starve the engine for fuel, causing it to run lean, misfire, hesitate or stall. Too much fuel pressure can cause the engine to run tough, waste fuel and pollute.
Electric fuel pumps are usually mounted inside the gasoline tank, though some may be mounted outside the tank. Some cars may even have two pumps (a transfer fuel pump inside the tank, and a main pump outside). The in-tank location helps muffle the buzzing noise produced by the electric pump motor, and immersing the pump in fuel helps lubricate and cool the pump motor. Driving with the fuel tank less than 1/4 full can shorten pump life by causing it to run hot. It also increases the risk of momentarily starving the pump for fuel when cornering sharply, braking or accelerating. Running out of fuel can sometimes damage an electric fuel pump by starving it for cooling and lubrication.
The pump is usually part of the sending unit assembly, that includes a float that sends an electrical signal to the fuel gauge on the instrument panel. If an electric fuel pump needs to be replaced, it can be replaced as a separate item or as a complete module assembly (which is more expensive but easier and less troublesome).