A car's starter motor is an electric motor that engages and turns the main crankshaft, drawing air and fuel into the cylinders so the engine can start. To do this, the starter draws heavy current through the battery, engages a flywheel or flexplate connected to the crankshaft, and turns it.
For an internal combustion engine to start, the cylinders need to be filled with the proper mix of air and fuel for combustion. In early vehicles, a physical crank, usually on the front of the automobile, accomplished this. After a few early drivers were run over by their own machines, an electric starting system replaced the crank to allow the driver to start the engine from the safety of the vehicle.
When the driver turns the key in an automobile's ignition, this engages the starter motor. Under normal circumstances, the starter is disengaged from the engine, but the starter solenoid pushes a gear forward to mesh with the flywheel in a standard transmission or a flexplate in an automatic transmission. The starter turns the engine, allowing it to begin the combustion cycle and run under its own power. Once the engine is turning, the key is released and the starter solenoid withdraws the gear.