http://www.threadsmagazine.com/item/...stitch-is-made
This is the site, tells how a stitch is made, but also shows just exactly what a bobbin does.
I used my embroidery machine yesterday (and will again today)for a couple projects, two the the designs used 2-1/2 bobbins of thread. Major irritation = running out of bobbin thread, particularly when it is the same color as the top thread.