The Great Link is a lake-sized body of Founders and the executive body of the Dominion. It consists of many Founders (thousands, millions, possibly billions) merged into a single body that shares knowledge and experience. This "gestalt" of Founder bodies and minds makes major decisions for the Dominion.
Founders returning from covert operations typically merge with the Great Link to share the intelligence they acquired during their missions. Starfleet's Section 31 was able to exploit this behavior to deliver a genetically engineered virus to the Founder population. This virus nearly exterminated the Founders.