[jQuery] document.getElementById('elid') != $('#elid') ?

[kitsched]

I was trying to populate a select's options when I noticed that I can
use:

sel = document.getElementById('product_id_01');
sel.options.length = 0;

but I can't use:

sel = $('#product_id_01');
sel.options.length = 0;

If anyone could provide a short explanation I'd be really grateful.

Thank you.