A traditional alternator uses a coil that uses DC power from the battery to produce a magnetic field. This coil is wound on a spool and is sandwiched between two metal flywheels with fingers of metal that alternate from one side to the other.
When this coil is powered, it makes each finger have a magnetic field. These alternate with each finger between north and south pole. As this assembly spins, the alternating poles of magnetism energize the outer windings of the alternator to produce AC power.