The Changelings were a liquid-based shape-shifting species native to the Gamma Quadrant. They served as the leaders of the Dominion and were referred to as the Founders by the various Dominion species, who revered them as gods. The majority of Changelings were joined together in the Great Link.
In 2372, Section 31 created a morphogenic virus and infected Odo, who unknowingly then infected the Great Link. By 2375, the virus had progressed so far as to threaten the Changelings with extinction. A cure was devised by Dr. Julian Bashir, and after the end of the Dominion War, was passed to the link by Odo. (DS9: "Extreme Measures", "What You Leave Behind")