As per topic, what actually differentiates a generator and alternator?
One generates and one alternates... :)
A generator produces DC directly, an alternator produces AC which is then converted to DC with a bridge rectifier.
A generator is a generic term for a device which, broadly speaking, converts mechanical to electrical energy. An alternator is a type of generator which works as described by Integral, and can be contrasted with a dynamo, which directly produces DC.
Separate names with a comma.