What is a GDS (in the context of the airline industry)?
GDS stands for Global Distribution System:


  A Global Distribution System (GDS) is a network operated by a company
  that enables automated transactions between travel service providers
  (mainly airlines, hotels and car rental companies) and travel
  agencies. Travel agencies traditionally relied on GDS for services,
  products &amp; rates in order to provision travel-related services to the
  end consumers. A GDS can link services, rates and bookings
  consolidating products and services across all three travel sectors:
  i.e., airline reservations, hotel reservations, car rentals.


These are the databases that maintain travel reservations and allow travel agents to see available inventory and book tickets. As noted in the Wikipedia article, they may also host airlines' computer reservations systems and passenger service systems. For example, Amadeus operates a system used by hundreds of airlines, not to mention thousands of hotels, rental car companies, tour operators, and other travel businesses to maintain reservations and track inventory, and a global distribution system used by tens of thousands of travel agencies, ranging from storefronts to online booking websites, that allows agents to book travel with these providers.