In commercial aviation, the first officer is the second pilot (sometimes referred to as the "co-pilot") of an aircraft. The first officer is second-in-command of the aircraft, to the Captain who is the legal commander. In the event of incapacitation of the captain, the first officer will assume command of the aircraft.
Control of the aircraft is normally shared equally between the first officer and the captain, with one pilot normally designated the "Pilot Flying" (PF) and the other the "Pilot Not Flying" (PNF) for each flight. Even when the first officer is the flying pilot, however, the captain remains ultimately responsible for the aircraft, its passengers, and the crew. In typical day-to-day operations, the essential job tasks remain fairly equal.
The pilot is seated in the left seat of the cockpit, and the co-pilot is positioned in the right seat.
Generally one of the two will fly the airplane, while the other navigates and is in charge of radio communication.