What's the difference between a cdl class a and a cdl class b?

Class A: Vehicles (or combinations) with a GVWR/GCWR in excess of 26,000 lbs., provided the vehicle in tow (i.e., the trailer) has a GVWR in excess of 10,000 lbs.

Class B: Single vehicles with a GVWR in excess of 26,000 lbs., or combinations with a single vehicle with a GVWR in excess of 26,000 lbs, and a vehicle in tow with a GVWR not exceeding 10,000 lbs.