The starter motor is an electric motor that rotates your engine in order to allow the spark and fuel injection systems to begin the engine's operation under its own power. Typically, the starter is a large electric motor and stator coil mounted to the bottom (generally to one side) of the vehicle's transmission bell housing where it connects to the engine itself. The starter has gears which mesh with a large flywheel gear on the back side of the engine, which turns the central crank shaft. Because this is a lot of physical weight and friction to overcome, starter motors are generally powerful, high-speed motors and use an ignition coil to ramp up their power before engaging.
How Starters Work
Starter motors of various types have been in use almost since the combustion engine was invented. Starting a typical automotive or aircraft engine by conventional, physical means (turning a crank or propeller, in most cases) was an inconvenient, dangerous, and physically demanding job. The first electric starter to appear on an automobile was in 1896 on an Arnold and became standard on Cadillac models in 1912. They were standard on nearly all automobiles sold by 1920. Chrysler added the key switch we're familiar with today in 1949, before which starters were usually button-activated.
The starter operates as part of the engine's starter circuit or ignition circuit. It is supplied power from its ignition coil (starter solenoid), which receives power from the vehicle's battery and amps it up to give it in a sudden burst to the starter. The starter itself is a powerful electric motor, either of permanent-magnet or series-parallel wound type. The amount of power fed to it would easily burn it out after a few minutes of operation, but allows it to safely crank very hard for a few seconds at a time.
In most vehicles, current from the solenoid that powers the starter also simultaneously engages a lever that releases the drive pinion, meshing the pinion with the starter ring gear on the flywheel of the engine. When the engine is running, this pinion is disconnected, allowing the gear to turn freely without affecting the engine's operation or the starter itself. In some engine configurations, specifically various forms of hybrid systems, the starter motor is used as a power generator when the engine is running, converting the spinning flywheel energy into electricity to feed electrical systems or batteries on the car. The Toyota Hybrid Synergy Drive system operates in this way.
Problems with Starters
Typically, the problems associated with starter motors and starting the vehicle are not the motor itself failing, but one of the electrical components or physical gears having failure. Often, the solenoid will become resistant or lose a connection and become incapable of powering the starter with sufficient force. Sometimes, the pinion gear fails and the starter cannot connect to the flywheel to turn over the engine. Rarely does the starter itself physically fail before the vehicle's life span is up.