The shapeshifting Changelings of the Gamma Quadrant who rule the Dominion from behind the Vorta and Jem'Hadar.
The Founders are the ruling class of the Dominion, a species of shapeshifters known as changelings who can take any form they wish. In their natural state they are a fluid, liquid mass, and they are able to blend into their surroundings, which makes them very hard to detect.
The Founders were once persecuted by solids, as they call other species, and this experience led them to build a system of control in which they bring other peoples under their rule through the Vorta and the Jem'Hadar. They link with one another in the Great Link, a shared state of mind and body that is central to their society.
The Founders appear throughout Deep Space Nine, and the best known is Odo, the station's security chief, who was found as a changeling and who gradually discovers his origins. The female Founder, played by Salome Jens, leads the Dominion during the war against the Federation and its allies. The conflict ends with the Founders surrendering, and Odo returns to the Great Link to help to heal his people.