In the most basic sense, tracks are the different types of information within a movie. Inside one QuickTime movie you could have a video track, a sound track, a voice track, and a title track. You may even have more than one of each. You could make a QuickTime movie with two video tracks, three voice tracks, and one sound track.
Tracks allow the creator of the movie to decompose the movie into different components.
This will give flexibility to the programmer. For example, if a director completes a movie, but is not quite satisfied with the way an actor's voice is in one scene, then the director could adjust that particular track. This is alot simpler than remaking the entire video. Unfortunately, there is no one way to work with tracks. Different editing programs have different ways of performing this task.