--refers to the cynadie chemical radical that is attached to the cobalamin portion of the B12 molecule in most commercial B12 preparations.

This is the cheapest way to supplement cobalamin (which, by the way, is so named because it has a cobalt atom in its chemical structure), and while cyanocobalamin supplementation does work for many people with B12 deficiency, there are some who have trouble chemicaly removing the cyanide radical and then methylating the cobalamin portion in order to make it usable by the body. Such people should always take methylcobalamin or other activated B12 form.

And, since methylcobalamin is now readily available and not really expensive, there's no reason those of us who need supplementation, or merely want it, to not get it--who wants a cyanide radical running around the body anyway?