I don't think I'll be very effective in trying to explain how an alternator works here, but I'll give it a try.
Refer to this Wikipedia entry for how they really work haha:
Alternator
An alternator is an energy converter. It converts mechanical rotational energy into electrical energy using the properties of magnetism. By passing a coil of wire through a magnetic field, an electrical potential is induced in that coil of wire.
Early designs used permanent magnets to provide the magnetic field. These were called
generators.
Alternators use another coil of wire to create the magnetic field, along with a bit of electricity from outside to get the process started. This electrical energy is provided by a (the) battery.
In order to control, or
regulate the amount of electricity needed
out of the alternator it's not always practical to vary the rotational speed so the magnetic field is regulated by a separate device.
Conventional alternator designs have historically placed this voltage regulator inside the alternator. This would be designed to provide a constant output voltage to run the car, charge the battery, etcetera.
By removing the voltage regulator from the alternator and giving control of that function to the vehicle ECU, the mechanical power used to operate the alternator can be better managed for a variety of reasons, such as efficiency gains, automatic start/stop operation, and even regenerative braking.