from wiki....
Code sharing or codeshare is an aviation business term for the practice of multiple airlines selling space on the same flights, where a seat can be purchased on one airline as if it was actually operated by a cooperating airline under a different flight number or code. The term "code" refers to the identifier used in flight schedule, generally the 2-character IATA airline designator code and flight number. Thus, XX123, flight 123 operated by the airline XX, might also be sold by airline YY as YY456 and by ZZ as ZZ9876. It allows greater access to cities through a given airline's network without having to offer extra flights, and makes connections simpler by allowing single bookings across multiple planes. Most major airlines today have code sharing partnerships with other airlines and code sharing is a key feature of the major airline alliances.
Under a code sharing agreement, the airline that actually operates the flight (the one providing the plane, the crew and the ground handling services) is called the operating carrier. The company or companies that sell tickets for that flight but do not actually operate it are called marketing carriers or validating carriers.