The CDL Manual explains license class in the introduction. There are three classifications: A, B and C.
License Class A is sometimes called the combination class. It means you can drive any combination of heavy vehicles. With Class A you can drive any vehicle weighing more than 26,000 pounds, and you can tow a second vehicle or trailer weighing more than 10,000 pounds.
Your license may also have endorsements or restrictions. Endorsements are like options to your license class. The Passenger endorsement permits you to drive a small Class C bus or a larger Class B bus. The Tank Vehicle and Hazardous Materials endorsements are needed to drive a tank truck carrying flammable liquids.
Aim for Class A. It gives you the most career opportunities. Add endorsements that you think you might need. You can always add an endorsement later.