Extra parentheses on function

Possible Duplicate:
What do parentheses surrounding a JavaScript object/function/class declaration mean?
What does this “(function(){});”, a function inside brackets, mean in javascript?
A Javascript function

I encountered markup similar to this:

var something = (function(){
    //do stuff
    return stuff;
})()
document.ondblclick = function(e) { alert(something(e)) };

I don't understand the opening ( and closing )() in the something variable. Could you explain the difference to writing it like this?

var something = function(){
    //do stuff
    return stuff;
};

Thanks!