MTB implies that they are primarily for mountainous terrain, which I think is a misnomer. I think all terrain is more accurate, at least from a C&V perspective. These days, there are all kinds of sub-categories/disciplines (free-ride, x-country, downhill, etc.) Maybe I'll start calling them ORBs (Off Road Bicycles) and confuse the heck out of everybody.

I think the one thing that really drove home my personal distinction was attending a "mountainbike race" at Lake Placid. Me and my buddy drove down, expecting some really grueling climbs, as we had previously competed in several road races that included Whiteface Mtn. However, when we got there, we looked at each other in disbelief. The course was a small, criterium style circuit at the base lodge. It was off-road but certainly wasn't mountainous, having maybe two feet of total climbing per lap! Needless to say, it was tailor made for a couple of ex-roadies.