Flash CS5.5 Self-Paced Course I

Movie Clips

Movie clips are a key element for people who create animated content with the Flash authoring tool and want to control that content with ActionScript.
Whenever you create a movie clip symbol in Flash, Flash adds the symbol to the library of that Flash document.
When an instance of a movie clip symbol is placed on the Stage, the movie clip automatically progresses through its timeline (if it has more than one frame) unless its playback is altered using ActionScript.*

Adobe TV: Understanding Movie Clips

* Source: Adobe Flash Help files