Graphic: Use graphic symbols for static images and to create reusable pieces of animation that are tied to the Timeline of the main movie. Graphic symbols operate in sync with the movie's Timeline. Interactive controls and sounds won't work in a graphic symbol's animation sequence.
Button: Use button symbols to create interactive buttons in the movie that respond to mouse clicks, rollovers or other actions. You define the graphics associated with various button states, and then assign actions to a button instance. See Assigning actions to a button.
Movieclip: Use movie clip symbols to create reusable pieces of animation. Movie clips have their own multiframe Timeline that plays independent of the main movie's Timeline—think of them as mini-movies inside a main movie that can contain interactive controls, sounds, and even other movie clip instances. You can also place movie clip instances inside the Timeline of a button symbol to create animated buttons.
Straight out of the AS dictionary.