In short
Commercial Driver License — required to operate commercial vehicles in the US.
A Commercial Driver License (CDL) is required to operate commercial vehicles in the US, including trucks over 26,000 lbs, vehicles carrying 16+ passengers, and hazardous-materials transports.
Three classes: Class A (combination vehicles), Class B (single heavy vehicles), Class C (smaller commercial / passenger). Each requires a knowledge test plus a skills test with three parts: pre-trip inspection, basic vehicle control, and on-road driving.