What are the essential elements of any generator?

If you are talking of a power generator, it is mainly a stator and a rotor and excitor. To house all of these, there will be a cover and to rotate a generator there will be an engine coupled with the generator. To control operations of the generator, a panel is also provided which has some voltage, ampere and/or frequency indicators, at times generator speed is also indicated on it. Bigger generators with provision for hook up with other generators also have separate panel for synchronization with other generators or power distribution systems.