The drop becomes the ocean.
And if you choose to take solid form again?
The ocean becomes a drop."
The Great Link (or The Link) was the intermingling of many Changelings in their natural liquid form, and the foundation of their society, providing meaning for the Changelings' existence. (DS9: "The Search, Part II")
As the home of the Founders, the Link was effectively the "capital" of the Dominion. For millennia, the Link was located on a rogue planet within the Omarion Nebula, until it was discovered by Odo and the crew of the USS Defiant in 2371. This lead the Tal Shiar and the Obsidian Order to launch a joint assault on the Founders' homeworld, intending to wipe out the Founders and destroy the Dominion. However, the entire attack was actually a plot to weaken the Cardassian and Romulan Empires – the Founders had already evacuated the planet, and a fleet of Jem'Hadar fighters was waiting inside the nebula to annihilate the invaders. (DS9: "The Die is Cast")
In 2372, the Founders forced Odo to return to the Great Link, then relocated to the Founders' new homeworld, for judgment after he murdered another Changeling. As punishment for his crime, Odo's shapeshifting abilities were taken away, and he was given Human form. (DS9: "Broken Link")
In 2375, the Changeling Laas revealed that he was one of the Hundred Changelings who were sent by the Founders to gather information on the solids they encountered. Like the Founders, Laas had an established distrust of solids based on experience. However, he would set out to find the other Hundred and create a new Great Link. (DS9: "Chimera")
Unknown to the Founders at the time, Odo's temporary joining with the Link had infected all of the Changelings with a morphogenic virus that began to inhibit their natural shapeshifting abilities, slowly killing them. A cure was later brought back by Odo, who single-handedly cured the entire Great Link, an event which was instrumental in ending the Dominion War. After curing his people, Odo decided to stay with the Link. (DS9: "What You Leave Behind")
According to the script for DS9: "To the Death", when Weyoun claps Odo on the shoulder, he introduced the illness that presents itself in "Broken Link" and necessitates Odo's return to the Great Link (where Odo introduces the Section 31 virus to the Great Link). This fact, however, is never obvious on screen.
Ronald D. Moore stated, "The Great Link rules the Dominion and I think the Female Shapeshifter has said that there is very seldom any dissension in the Link. She is not the leader and presumably they all share power jointly." (AOL chat, 1997)
During production on "The Search" two-parter, there was only a verbal description of what the Great Link would look like. The actors consequently had to utilize all their professional abilities to make it credible. (Star Trek: Deep Space Nine Companion, p. 167)