GDS  is the Global Distribution System. An international computer Reservation System that accesses data from many Suppliers (airlines, etc.) across many countries.

Other definition:
GDs are a network of Internet reservation systems that provide a central place where travel agents and travellers can check availability and reserve travel related products such as hotels, airline and car rentals. These systems are formed and managed by the airline industry and includes system such as Amadeus, Sabre, Apollo and Pegasus.
Other definition:
GDS are the A massive computerized reservations system that accesses international databases of airlines, hotel chains and car rental companies. Used by traditional travel agents and is now the backbone of major online travel agencies.
List of books: GDS