It's one thing to get an airplane into the air. It's another thing to control it effectively without crashing back to earth. In a simple light airplane, the pilot transmits steering commands via mechanical linkages to control surfaces on the wings, fin and tail. Those surfaces are, respectively, the ailerons, the elevators and the rudder. A pilot uses ailerons to roll from side to side, elevators to pitch upward or downward, and the rudder to yaw port or starboard. Turning and banking, for example, requires simultaneous action on both the ailerons and the rudder, which causes the wing to dip into the turn.
Modern military and commercial airliners have the same control surfaces and take advantage of the same principles, but they do away with mechanical linkages. Early innovations included hydraulic-mechanical flight control systems, but these were vulnerable to battle damage and took up a great deal of room. Today, almost all large aircraft rely on digital fly-by-wire systems, which make adjustments to control surfaces based on an onboard computer's calculations. Such sophisticated technology allows a complex commercial airliner to be flown by just two pilots.