The Global Distribution System is a large centralized system that gives rates to travel sites, travel agents, airlines, and car rental agencies. The rates are either used for business for corporate travel or the ordinary leisure traveler that uses a travel agent to book their vacation. Additionally, we are now seeing more and more large companies like Office Depot, Best buy, and other large ones using travel agencies to coordinate corporate travel for them by collecting a commission from the hotels. The following website will educate you on exactly what it does and answer any pending questions you're curious to.
https://openhotel.com/blog/globaldistributionsystems/post.cfm/post/5986/faqs-for-gds
http://www.tts.com/blog/why-is-gds-important-to-the-travel-industry/