GDS: A global distribution system (GDS) is a network operated by a company that enables automated transactions between third parties and travel agencies in order to provide travel-related services to the end consumers. A GDS can be used to make an air, hotel, car or other travel service booking. And not only do GDSs power the content of ‘traditional’ travel agency platforms, but they also provide pricing, availability and reservation functionality to many online travel agencies.

CSR: A computer reservations system or central reservation system (CRS) is computerized system used to store and retrieve information and conduct transactions related to air travel, hotels, car rental, or activities. Originally designed and operated by airlines, CRSes were later extended for the use of travel agencies. Major CRS operations that book and sell tickets for multiple airlines are known as Global Distribution System (GDS).
The premier global distribution systems are Amadeus, Galileo, Sabre, and World span.