A symbol is a reusable image, animation, or button. An instance is an occurrence of a symbol on the Stage or nested inside another symbol. Symbols can make editing a movie simpler as changes to repeating elements need only be made to the symbol and Flash updates all instances.
Using symbols in your movies dramatically reduces file size; saving several references to a symbol requires less storage space than saving a complete description of the element for each occurrence. For example, you can reduce the file size of your movies if you convert static graphics such as background images into symbols. Using symbols can also speed movie playback because a symbol needs to be downloaded to a browser only once.
For an interactive introduction to using symbols and instances, choose Help > Lessons > 3 Symbols.
A symbol in the library and two instances on the Stage