the only difference is that, in motors, electrical energy is input to create kinetic energy. and conversely, kinetic energy is input to create electrical energy in a generator.
Umm you still need a current for the electromagnets (or you could use permament magnets). So you need two circuits (whereas in a DC motor you can run both off the one circuit).
An AC motor can't necessairly function as an AC generator depending on if you are using a Universal motor or not (which is basically a DC motor that uses electromagnets) they usually run on AC but would generate DC. Where as the ones with slip rings would generate AC.