[jQuery] Different between this and $(this)

[jQuery] Different between this and $(this)


Hi jQueries;
what is the different between this and $(this) ??
I try the follow lines
$("#form_zuw input[readonly]").each(function() {
this.addClass("input_readonly");
});
like
http://jquery.bassistance.de/jquery-getting-started.html
$(document).ready(function() {
    // use this to reset several forms at once
    $("#reset").click(function() {
        $("form").each(function() {
            this.reset();
        });
    });
});
and Firebug say "addClass is not a Function"...???
and this is running
$("#form_zuw input[readonly]").each(function() {
$(this).addClass("input_readonly");
});
Can you explain the different?
Thanks
x-herbert