[jQuery] question about a javascript syntax

[Shuai Yang]

I found Jörn Zaefferer's "jQuery plugin authoring guide" at
http://bassistance.de/2006/12/27/jquery-plugin-authoring-guide/,
there i found javascript syntax like this:
========================
(function() {
 // some code here
})();
========================
(function($) {
 // some code here
})(jQuery);
========================
it really works,but i donno what exactly this syntax means,anyone can
explain it? great thanks!