javascript global variable with 'var' and without 'var'

Possible Duplicate:
Difference between using var and not using var in JavaScript

I understand that I should always use 'var' to define a local variable in a function.

When I define a global function, what's the difference between using 'var' ?

Some of code examples I see over the internet use

var globalVar = something;
globalVar = something;

What's the difference?