Specifically how does an auto alarm work?

An auto alarm is composed of one or more sensors connected to a siren or the car's horn. When those sensors detect certain types of motions, they activate the siren (or the horn) so it sounds a loud alarm, hopefully deterring the would be thief. The owner has the ability to override the alarm (and enter into his own car) by entering some sort of code, or pushing a button on a key fob.