CDL classes and endorsements, in plain terms
Class A vs. Class B
A Class A CDL is for combination vehicles — a tractor pulling a separate trailer, where the whole rig is rated over 26,001 lbs and the trailer alone is over 10,000 lbs. That's the license for over-the-road freight: dry van, reefer, flatbed, the 18-wheeler most people picture. A Class B CDL is for single-unit vehicles where the cab and the cargo are one piece: dump trucks, box trucks, garbage trucks, cement mixers, and most buses. The most common and most costly mistake we see is people getting a Class B believing it lets them drive a semi. It doesn't — and upgrading later means re-testing.
The tests almost everyone takes
Every applicant takes a General Knowledge test for their class. Class A adds Combination Vehicles. Nearly everyone also needs Air Brakes, because skipping it puts an air-brake restriction on your license that blocks you from operating almost every commercial truck.
Job-specific endorsements
Beyond the core tests, endorsements match what you'll actually do: HAZMAT (H) for hazardous materials, which also requires a TSA background check that can take a month or more; Tanker (N) for liquids or gases in a tank; Doubles/Triples (T) for pulling more than one trailer; Passenger (P) for buses; and School Bus (S) for school bus driving. Hauling fuel typically means HAZMAT and Tanker together, sometimes called the X endorsement.