The alternator is actually alternating current generator and its function is the production of electricity for the car (for all its electrical parts), primarily performed by charging the battery. We could say that the generator behaves like a small power plant on the vehicle. Today alternators are stronger than they were before, and already at a small speed to create enough energy to start the battery.
What is interesting is that the battery needed to operate the direct current, a generator produces alternating so that the generator still has a router that should convert AC power to DC. For this use LEDs or other types of routers. For this reason, the alternator got open road in vehicles only after the discovery of silicon diodes and transistors, and before they perform this function voltage regulators and routers that were very large and occupied a lot of space.
Today, both the generator automatically adjusts the presentation of their strengths and for that he needed a voltage regulator. In this story used and transistors that allow regulators become so small so they can be installed as early as I alternator.