[jQuery] Different between this and $(this)

[x-herbert]

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