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

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


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.