650B is in between a 29er and a traditional 26 inch mtn wheel. It's nice for people like me who feel like a 29er bike is just too dang big, the wheelbase too long for tight cornering being a primary issue.

The outside diameter of a 650 wheel with the tire on it is the same whether it is A, B or C. So 650C is a road wheel with the tire on that has the same outside diameter of a 650B mountain wheel. The tire sizes are different so the actual rim sizes are also different, the mountain rim is smaller in diameter, but the tire is bigger.

Clear as mud right?