Class creation problems

Hi… I m a bit confused with the object creation procedure in actionscript/Flash… One way is to do [COLOR=RoyalBlue]var[/COLOR] myCircle:Cirlce = [COLOR=RoyalBlue]new[/COLOR] Circle(); (provided you have a Circle class defined) and take it from there… the other is to right click on the Movie Clip you want to become an instant of the Circle Class and do - Linkage - Export for Actionscritp and give it the Class name…

What is the difference? What does the second method do in terms of code?
How come Private variables are not so private in the second method and not in the first where the compiler will properly throw an error if you try to access them?

Any suggestions?