a short list of built-in movie clip events
In order to attach a movie clip event handler you must first select an Instance on stage.
Whereas Frame and Button events are inside of a timeline, Movie Clip Events are attached to the outside.

onClipEvent( event ){ ... }

EVENT TRIGGER
load Event occurs once, the moment when the movie clip appears on stage for the first time.
A good place to initialize variables for a movie clip.
enterFrame Event occurs repeatedly, once every frame while a clip is on stage.
A good place to execute code that need to be updated constantly.
mouseDown Event occurs every time the primary mouse button is depressed, anywhere on stage, while the clip is on stage.
mouseUp Event occurs every time the mouse is released, anywhere on stage.
mouseMove Event occurs every time the mouse is moved any amount, anywhere on stage.
(caution: memory hog)
unload Event occurs once, the moment after the movie clip is removed from the stage.
A good place to execute clean-up code.

Example: Using ActionScript to move things around:

onClipEvent(enterFrame){
    _x = _x+5
}

Example: Using ActionScript to set a variable:

onClipEvent(load){
    myXlimit = 550
}