A symbol is a graphic, button, or movie clip that you create once in the Flash Professional authoring environment or by using the SimpleButton (AS 3.0) and MovieClip classes. You can then reuse the symbol throughout your document or in other documents.
A symbol can include artwork that you import from another application. Any symbol that you create automatically becomes part of the library for the current document.
An instance is a copy of a symbol located on the Stage or nested inside another symbol. An instance can be different from its parent symbol in color, size, and function. Editing the symbol updates all of its instances, but applying effects to an instance of a symbol updates only that instance.
Using symbols in your documents dramatically reduces file size; saving several instances of a symbol requires less storage space than saving multiple copies of the contents of the symbol. For example, you can reduce the file size of your documents by converting static graphics, such as background images, into symbols and then reusing them. Using symbols can also speed SWF file playback, because a symbol needs to be downloaded to Flash® Player only once.
Share symbols among documents as shared library assets during authoring or at runtime. For runtime shared assets, you can link assets in a source document to any number of destination documents, without importing the assets into the destination document. For assets shared during authoring, you can update or replace a symbol with any other symbol available on your local network.
If you import library assets with the same name as assets already in the library, you can resolve naming conflicts without accidentally overwriting existing assets.
Additional introductory instruction about symbols is available from these resources: