A generator that transforms mechanical energy into electrical energy, specifically AC Power (alternating current). These are usually found in automobiles. They make it go by charging the battery and maintaining an electrical current to one or more of the following items: a) spark plugs, b) air conditioning, c) stereo, d) power windows, e) various pumping mechanisms... etc.
The alternator in my car is broken. My battery has no juice even right after I shut off the car.