ActionScript for Flash MX: The Definitive Guide, 2nd Edition By Colin Moock
Part I: ActionScript Fundamentals
You've learned a lot about composing instructions for the ActionScript interpreter to execute. By now you should be pretty comfortable telling the interpreter what you want it to do, but how do you tell it when to perform those actions? ActionScript code doesn't just execute of its own accord something always provokes its execution.
That "something" is either the synchronous playback of a movie or the occurrence of a predefined asynchronous event.
