A generator is a device that converts mechanical power into electric power. A dynamo is a generator that uses a commutator to produce pulsed DC. An alternator is a generator that produces AC (which may later be rectified & filtered to DC). The term "alternator" is usually restricted to small generators used in automotive applications, larger AC generators such as those in power plants get described more precisely, e.g. induction generators or self-excitation generators. There are some other less-common generator types, such as homopolar generators that output continuous DC.