The main difference between ATPL (Airline Transport Pilot License) and PPL (Private Pilot License) is their purpose and requirements. A PPL allows you to fly privately, while an ATPL is for pilots who want to command commercial airlines. To get an ATPL, you need more flight hours, advanced training, and exams, compared to the simpler requirements of a PPL. ATPL holders can work as captains for commercial airlines, while PPL holders cannot fly for hire or as a career pilot.