Symbols are by far the most important aspect of Flash. The ability to have separate timelines and reuse library items is what the application is all about. All symbols have their own timeline separate from scenes. All symbols are stored in the library when made.

Symbols decrease file size, create prototype objects which can be controlled with programming, and save time by storing your artwork.

To make a symbol out of selected artwork on the Stage (keeps the symbol on the stage and puts in library):
Select artwork and press the hotkey to make a symbol: F8
or
Modify>Convert to Symbol

To create an empty new symbol to put artwork in (puts a symbol in the library which must be dragged out to the stage from the library panel):
Insert>New Symbol

You will choose the symbol type from the dialog box that follows. NAME YOUR SYMBOLS in a way that makes sense. The different symbol types are: graphics, movie clips and buttons.

Button: For interactivity. 4 frame timeline: up, over, down, hit.

Graphic: can be animated, play in step with timeline in which they appear. They are dependent on the playhead being above all keyframes that they occupy and will not play their entire timeline if the playhead is kept still. You do not need to publish the movie outside of the Flash application to see it animate. They cannot have sound in their timeline or programming and cannot be controlled with programming.

Movie Clip: can be animated, can have sound in timeline, can be controlled with programming, have independent timelines to main scene timeline. A movie clip plays continuously when the playhead is above a keyframe it occupies. It will play the entirely of the movie clip and loop as long as this is true, even if the playhead is stopped on the timeline in which the movie clip appears. But, you need to publish the file to see a movie clip animate.

What is an instances?
There is a difference between symbols in library and symbols on the stage. You can have as many instances of a symbol on the stage as you like, but they all refer to the same symbol in the library. This way you can change the symbol and all will be changed. Every instance on the stage of a symbol is unique and has its own properties (position, alpha, tint, scale, rotation, etc.).

For example, lets say you are making rain and must turn it into hail or snow. You can make a rain drop, drag many onto the stage in different positions to create the rain. Then if you want to change it to hail or snow, just edit the one symbol in the library, change the one piece of artwork and everything will change to snow or hail on the stage. Maybe this seems unusual, but it isn't. Say a character has blonde hair and you have create a whole animation with this character and you need to change his hair to brown. Simply change the one symbol of the hair to brown and the whole animation will change to a character with brown hair (that is if you've used and reused only one symbol of a characters head with hair).

Note: You can change the behavior of a symbol in the Properties Panel. This does not effect the library item, just the instance of the symbol on the stage.

Symbol Timelines

Organizing animation by scenes or by movie clips is always a decision that should be considered. For most beginning to intermediate Flash users and programmers, Scene by Scene animation is easier, but as programming gets more advanced, it is wise to do all of your organization in movie clips so that they can be controlled through programming in one scene.

In general it is a good idea to use only movie clips and not graphics for symbols because not only can they be controlled with scripting, but also nesting animated movie clips make smaller files than nested animating graphic clips. Nesting refers to the process of putting a symbol inside another symbol, which is common.