There has been some standardization within states concerning Commercial driver licences. In Minnesota they are:
Class A - Commercial Combination Tractor/trailer (Lorry) over 26,000 lbs (over 11.8 metric tons)
Class B - Commercial Non combination Truck over 26,000 lbs with standard trailer (not lorry trailer)
Class C - Commercial Non combination over 10,000 lbs but less than 26,000 lbs with trailer.
Class D - Non-commercial, general licence under 26000 lbs with trailer.
Each level up qualifies you to operate at the lower level.
Each Class can then have endosements based upon usage, i.e. motorcycle, hazmat, air brakes, school bus, passenger(general bus), agricultural, etc. Each endorsement is a seperate test.
For example, I have a Class B licence with passenger endorsement and airbrakes endosement. I used to have a school bus endorsement, but let it lapse when I changed jobs. I do not need to have a commercial license for my current job (only a class D), but I keep it up to date just in case.