Classes of FAA Aeromedical Exams in the US
Class 3 medical certificates are for private pilot duties only. They have the least restrictive medical requirements and the certificates are generally good for 3 years for applicants under age 40 and 2 years for those 40 and over.
Class 2 medical certificates are for commercial, as well as some private pilot duties. This certificate would be required of "for hire" pilots such as crop dusters, charter pilots, corporate pilots, and anyone else who flies commercially. The certificate is good for 1 year for commercial activities and 2 or 3 years for private pilot use (depending on age).
Class 1 medical certificates are required for pilots of scheduled airliners in Captain positions. First Officers and Flight Engineers may only need Class 2 level depending on the particular airline's policies. Class 1 exams have the most stringent medical requirements and the certificate is good for 6 months for airliner duties. Like the Class 2 certificate, however, it is good for 1 year for other commercial activities and 2 or 3 years for private pilot duties.
Any person acting as pilot-in-command or other required crewmember of an aircraft (except for free balloons, gliders, and ultralights) must hold a current and appropriate medical certificate. This includes student pilots in solo flight as well as private, commercial, and airline pilots. What about Sport Pilot licenses?