[jQuery] Why is getElementById different from $('testid')?

[Geuis]

So I have this code to write a content to an iframe which works just
fine. I'm trying to rewrite it using Jquery though.

        var f=document.getElementById('testiframe');
        var doc = f.contentWindow ? f.contentWindow.document :
f.contentDocument ? f.contentDocument : f.document;
        doc.open();
        doc.write(scriptvar);
        doc.close();

In testing, I'm finding that these are different:

        1) var f=document.getElementById('testiframe');
        2) var f=$('#testiframe');

So #1 gives [object HTMLIFrameElement] if I alert().
#2 gives [object Object].

What is the difference between these two?