Before we start creating Flash content, we need to know the nuts and bolts that go into making these... In this article you will learn about some of the most commonly used objects (Symbols) and their properties. Even these Symbols are objects and hence are defined by a class with properties and functions specific to each...

Button Symbol ::
Button symbols are used to create interactive buttons. Button symbols are made up of four key-frames, also known as states and are related to user interaction and are labeled Up, Over, Down and Hit .
The Up state defines the button when the button is inactive, Over state when the mouse moves over it, Down state when the mouse button is pressed down on the button, Hit state defines the hit-area of the button or the effective button area.

MovieClip Symbol ::
Movie Clips are actual movies within a movie. These are used for animations that run independent of the main movie's Timeline. These are similar to actual flash movies and can contain Actions.

Graphic Synmol ::
Graphic Symbols are mainly used for static images that need to be re-used in a movie. They cannot contain actions or sounds. These do not play independent of the main timeline and hence playing animations through graphic symbols and syncing them would be a hassle.

You can get your hands-on by creating your own symbol. To do so, select Symbol from the insert menu (Insert > Symbol) or use the shortcut, Ctrl+F8
and the window that opens, select the symbol you wish to create. 
In case of any confusion regarding timelines and frames, try this article.. Flash TimeLine