The Founders are the creators and rulers of the Dominion, an interstellar government that controls much of the Gamma Quadrant. A large-but-unspecified number of Founders join to form the Great Link, which is effectively the ruling body for the Dominion. The Founders are xenophobes who seek to control all "solid" species that they contact to prevent the repetition of anti-Changling atrocities alleged to have occurred in their past.
The Founders handle little of the day-to-day operations of the Dominion. The Great Link sets policy for the Dominion, but the Vorta perform most administrative functions. Individual Founders conduct covert operations in foreign territory, where they gather intelligence and attempt to destabilize enemy governments.
Changlings can change their shape and density with ease to imitate objects or living creatures, but the natural state of the Founders is liquid, and they must resume a liquid state periodically to remain healthy. When interacting with other races, they typically adopt a humanoid form. Unless actively disguising themselves, however, they generally leave out some of the details of the face, resulting in a rather "unfinished" appearance. Detailed facial structures are more difficult for them to imitate -- Odo has never mastered it, although many other Founders have. Experienced Founders mimic more than just the physical appearance of other life forms: they imitate their biological structures well enough to fool a tricorder. They can not, however, maintain control over portions of their bodies which have become physically separated; a small volume of a Founder removed from the main body will quickly revert to a liquid state.
In their liquid state, individual Founders are able to merge into the single, vast ocean of Founders that they call the Great Link. The Founders can relocate the Great Link to any hospitable planet, as they did after they lead the Tal'Shiar and Obsidian Order into a trap by leaking the location of the Great Link to the two spy organizations. The current location of the Great Link is unknown.
Founders can apparently change their mass when mimicking objects smaller than humanoids. Such a change would theoretically release energy on a thermonuclear scale, but no such energy-state changes are seen when Founders change their size. Naturally, the official explanation for this phenomenon involves subspace.