The starter motor rotates the engine until it can rotate on its own power. When a vehicle is started, current flows from the closed ignition switch to the starter solenoid.
Some vehicles use a park/neutral safety switch or clutch safety switch in this circuit and will only allow current to the starter if the automatic transmission is placed in Park or Neutral or the clutch pedal is pressed.
When the starter solenoid is activated, it pushes the starter pinion gear outwards so that is engages with the flywheel ring gear. At the same time, it also allows current to the starter motor, causing the pinion gear to spin.
The spinning pinion gear rotates the flywheel until the engine can rotate on its own.
