A symbol is an art object that you can reuse in a document. For example, if you create a symbol from a flower, you can then add instances of that symbol multiple times to your artwork without actually adding the complex art multiple times. Each symbol instance is linked to the symbol in the Symbols panel or to a symbols library. Using symbols can save you time and greatly reduce file size.
With Dynamic Symbols, symbols can share a master shape but different instances of the symbol can have dynamically modified appearances. If the master shape is modified, symbol instances receive the new modification, but maintain their own modifications as well.
Symbols also provide excellent support for SWF and SVG export. When you export to Flash, you can set the symbol type to MovieClip. Once in Flash, you can choose another type if necessary. You can also specify 9‑slice scaling in Illustrator so that the symbols scale appropriately when used for user interface components.