A class A CDL license allows you to drive any combination of vehicles weighing 26,001 or more pounds provided the vehicle(s) being towed are in excess of 10,000 pounds.
A Class A CDL is also needed to drive Semi's across state lines.
A Class B CDL License is a license that allows a person to operate any heavy straight vehicle with a gross vehicle weight rating that's greater than 26,000 pounds, as well as any vehicle towing another vehicle that is not in excess of 10,000 pounds.