Are reusable static images that are used mainly to create animations.
Any drawn vector/plain text/imported bitmap (photo), or combinations of these, can be converted into a single controllable object: as a graphic symbol.
They have their own non-restricted Timeline (any number of layers and frames)
The best thing about using movieclips is that you can control them from actionscript - you can change their dimensions, position, color, alpha, and other properties and can even duplicate and delete them.