You need to learn these four things to be able to make Flash projects:

What symbols are?

How to create them?

What are the three Symbol Types?

Advantages of using symbols?

What are Symbols?

A Symbol is a reusable object used in Flash. It is stored in the Library. You can drag-and-drop symbols from the Library to the Stage. Think of the library as a place you keep all of the parts for your flash project: pictures, movies,sounds, graphics, etc.

A copy of the original Symbol is called an Instance of that Symbol. An instance is kind of like a Shortcut in PC land.
Something to remember is if you edit one Symbol, all of its Instances will be updated as well. Changing the properties of an Instance of a Symbol does not effect other Instances.

How to make Symbols?

Creating Symbols is easy. It can be done in two steps:
1. Select an object in your Flash movie and press F8.
2. In the "Convert to Symbol" dialog box that will pop up, you can specify our Symbol's:
Name,Type, and Registration point.

Give it a name that will help you remember what it is and does. If it is a back button, name it back button. The name will help you locate it in the Library. Choosing the Type depends on the purpose you want to use your Symbol for. Read below for type information.

The three Symbol Types:

When converting an object to a Symbol, we can choose between the three Types:

Movieclip - A movieclip is like a small, self contained animation

Button - I button can have a script attached to it. The script can control the playback head in the timeline.You can have it change when you mouse over and click on it.

Graphic - Pictures and shapes.

Each of them is used for different things in Flash.

Why use Symbols?

1. Symbols can be tweened (you create the first and last frame in an animation sequence and the computer creates the in between frames).
2. Symbols don't get mixed. Thanks to that you can keep many Symbols on the same Layer.

Example of mixed objects ; If you have 2 objects overlap, when you move one it takes a bite out of the one underneath it. If you convert the objects to symbols first, that won't happen.

:

All symbols are automatically saved to the library. If you select a symbol in the list in the bottom of the Library pallet, it will show you the symbol in the top portion of the pallet.

Additional info about symbols:

1. Symbols can easily be swapped, moved, scaled up, scaled down. Just select the Symbol and press Q to transform it.
2. Button and Movieclip Symbols can be controlled via Actionscript. Graphic Symbols can't.
3. Using Symbols allow you make your movie smaller in file size.