An alternator is used to produce electrical current or power. In the case of the automobile, the alternator is powered by a drive belt that is attached to a pulley on the end of the engine's crankshaft, the shaft through the bottom of the engine that drives the entire engine as it turns. As the drive belt turns the alternator pulley, the alternator creates the electrical current that the car runs on; the battery is not used at all once the car is running and the alternator is functioning, except as a reserve energy source.
Inside the alternator's casing is the stator, a ring that is wound with copper wire. As the drive belt turns the alternator pulley, the rotor - a magnet that rotates within the stator - is rotated. This rotating magnetic field generates electrical current within the wound stator. Most automotive alternators also contain brushes and slip rings, which also help generate and control the current.
Alternator rewinding is the rewinding of the stator. Alternator rewinding may serve one of several purposes. Although it's more common for the brushes in an automotive alternator to fail, rebuilding an automotive alternator may require alternator rewinding. Alternator rewinding can also be used to upgrade the alternator for production of a higher power output; cars with powerful stereos or aftermarket ignition systems that draw a good deal of power may require an upgrade through alternator rewinding to avoid placing a heavy draw on the battery.
Another use for alternator rewinding is the conversion of automotive alternators for other purposes, such as a wind generator. Applications such as this require such different energy outputs that alternator rewinding is necessary in order to make use of an automotive alternator. However, the use of an automotive alternator is so much cheaper than other alternatives that alternator rewinding is a price many are willing to pay.