Alternators are so named because they produce
alternating current. It is sometimes more desireable to
produce alternating current because it is a
mechanically simpler thing to do. You can simply
rotate a permanent magnet in a set of field coils, rather than having to rotate the coils in a magnetic field, requiring a
commutator.
There are many different kinds of alternators, ranging from the tiny one in your car to the giant ones at your local power plant. Depending on how many poles an alternator has, it will produce that many number of phases. Three-phase power is produced using a three-pole alternator with each pole at 120 degrees.
The rotation rate of an alternator must remain constant if the current produced is to have a constant frequency. This isn't an issue in your car, since the power is rectified and filtered by the battery anyway, but in a power plant, all the power produced must be 60Hz (or 50hz if you're not in the United States). Thus, an alternator in this sort of duty has two modes: Spinning at the correct speed, or off. Thus, overloading is a bad idea, since the more energy consumed, the more torque is required to maintain a specific rotation rate. Alternators in mains duty tend to have familiar rotation rates, such as 1800 or 3600 RPM. Note that these rates divide evently into 60.
Alternating current is handy because you can voltage-convert it efficiently using transformers. If you want direct current, you can employ a dynamo.