Prime Mover:

The prime mover is the component which is used to drive the AC generator.  The prime mover might be any type of rotating machine, like as a diesel engine, a steam turbine or a motor.

Rotor

The rotor of an AC generator is the rotating parts of the generator, as display in above Figure. The rotor is driven through the generator's prime mover, that might be a gas turbine, steam turbine, or diesel engine.  Its Depending on the type of generator, this component might be the armature or the field.  If the voltage output is generated there the rotor will be the armature; rotor will be the field if the field excitation is applied there.

Figure: Basic AC Generator