Airline Transport Pilot
This is the highest certificate you can get as a pilot and is required to fly as a crew member in 121 operations (airline pilot) or to act as the pilot in command (PIC, [captain) of most multi crew aircraft.
In the United States you have to be at least 23 years of age and have at least 1,500 hours of total flight time before obtaining an ATP.
It is common to fly as a commercial pilot (flight instructor, first officer, etc.) until you have the needed hours and then you do the ATP/CTP course, written exam, flight course followed by a check ride. Today most all regional airlines will provide and pay for the ATP training.
