There are a few components inside an ALT that go bad. The rectifier or regulator are probably the most common to go. The stator or rotor are also possible items to go but are much more durable and less likely to go. On vehicles with higher mileage it could be as simple as worn brushes not making good contact anymore.
What causes these problems, sometimes wiring (grounding the output cable would be bad), Bad connections at the regulator (corosion) can cause failures, sometimes an over worked unit. Fften if one goes there is another underlying problem which caused it to go.