jQuery question: what does it really mean?

Question

(function($, window, undefined){
  ... jquery code... 
})(jQuery, window);

What does it really mean? Does it also mean $(document).ready()

Answer 1

(function($, window, undefined){
  ... jquery code... 
})();

is different than

$(document).ready() 

Paul Irish has a good video on 10 Things I Learned from the jQuery Source at 1:30 in he talks about the jquery source's self executing anonymous function and what the arguments mean