A class B has the van body with some extras. A class C only has the van cab with a motorhome built behind and on top.
B+ is when you get purists involved. They think a B-van is a simple conversion of a stock van to an RV and a B+ is when you add too much like a bit of extra length or too big a cap or something. For them, B+ is a term of derision as they think its too much like a class C which is too much like a Class A motorhome.
see
http://www.seventhheavenrec.com/content.asp?id=103&uid=