The Enclave are the genocidal post-war remnants of the original United States government in the Fallout video game series. They originated mainly from pre-war "shadow government" agents and other corrupt politicians, and consider themselves to be a legitimate continuation of the true U.S. Government. Their ultimate goal is to rebuild the American civilization in its original, pure form by eliminating all the "mutants" who are no longer fully human due to all the radiation. What the Enclave does not seem to fully grasp, however, is that these "mutants" include pretty much everyone except them and the surviving communities that are still locked away in vaults, and their goal would essentially entail genocide. In Fallout 2, the main goal is to stop the Enclave from releasing their mutant-killing F.E.V. virus into the wasteland, but in Fallout 3, the player is given the choice to whether or not inject this virus into the Capital Wasteland's water supply.
The Enclave were also secretly involved in the "Vault" program of nuclear shelters, which is one of the central elements of the Fallout universe. These vaults were ostensibly intended to protect those wealthy enough to afford a spot in one from the nuclear holocaust, but they were also used for a "social experiment" by the Enclave, where most of the vaults secretly had some unusual and often downright harmful "gimmick" intended to test how people would behave in such situations. For instance, Vault 11 supposedly required the inhabitants to sacrifice one of their own each year for the community to survive, Vault 106 was laced with mind-altering drugs that turned the people inside crazy, and Vault 112 was a virtual reality program where the Overseer controlling the simulation went insane and started torturing the other people for fun.
In Fallout: New Vegas, the Enclave has been almost completely destroyed, except for six now very old retired members who the player can reunite for one last mission during the endgame battle for Hoover Dam.
Fallout: New Vegas