Use of var

Hi All,
I have a basic doubt…

I have used global declaration like:

var a:String ="";
var b = 0;
var c:String = “”;

and I have a written a function called abc

function abc()
{
a="";
b= 0;
c= “”;
blah blah blah…!
}

abc(); //calling this function

I want to know the difference between the first declaration and the second declaration…Is there a difference or is it a re-declaration…?

Plz help…!
Thanks
Nirmal.