Car audio/video (car AV) is a term used to describe the sound and video system fitted in an automobile.

A stock car audio system refers to one that was specified by the manufacturer when the car was built. A custom car audio installation can involve anything from the upgrade of the radio to a full-blown customization of a car based around its audio equipment.

More description in details is written at the follow page.

http://en.wikipedia.org/wiki/Car_audio (Wiki)