They're the political/ideological organization that ruled the Aldmeri Dominion (Summerset and Valenwood) before Tiber integrated those lands into the Empire, and do so again after the decline of the Empire in the Fourth Era.
They represent the extremes of Aldmer beliefs, in that they're strongly opposed to Men and man's endeavors and want to return the world to the state of the Merethic Era. They think that creation is like a sort of prison, and that mortals are a step down from what they used to be.
http://www.imperial-library.info/content/pocket-guide-empire-first-edition-aldmeri-dominion is a description of the Dominion from the beginning of Tiber's reign, and you might search the Imperial Library for more info.