To begin the FMC is the Flight Management Computer. The FMS is the Flight Management System. The FMC is just a computer that calculates various things chiefly the navigation of the aircraft and is displayed on the CDU(control display unit). The FMC uses inputs from the FMS to do this. The FMS incorporates things such as the IRS the Air Data Computers and the FMC. Don't have my books infront of me so this is the best I can come up with.