From The TV IV
A laugh track is the sound of audience laughter included in many comedy shows. It can be recorded from the live audience present at a show's recording, or it can be "canned" (using a machine known as a MacKenzie to add pre-recorded laughter to the soundtrack). Even live recordings of audiences are sometimes manipulated to make the audience reaction seem stronger. The purpose of the laugh track is ostensibly to recreate the feeling of attending a live performance, but critics feel it's used to prompt the audience into laughing, hiding the fact that the program itself isn't very funny.
Most traditional sitcoms feature laugh tracks, and even some cartoons, like Scooby-Doo and The Flintstones, have had laugh tracks (despite the fact that audiences could not have been present for the recording of the show).