The CDL Manual explains license class in the introduction. There are three classifications: A, B and C.
Class A is the most flexible. It is also the most valuable class to have on your CDL. If you have a Class A license you can work at almost any driving job.
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.
License Class B is often known as heavy straight vehicles. This class allows driving any single vehicle with a gross vehicle weight rating (GVWR) of more than 26,000 pounds. Class B also allows towing a second vehicle up to 10,000 pounds GVWR.
License Class C is the small vehicle class. Class C includes any vehicle or combination that does not fit in Class A or Class B. It includes passenger vehicles for up to 16 people, including the driver. Small vehicles used to transport hazardous materials are also included in Class C.
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.