The central text, the Principia Discordia, first circulated in 1963-65 as a handful of photocopied pages, and it grew by accretion across later editions rather than being written as a book. Its authors, Greg Hill ("Malaclypse the Younger") and Kerry Thornley ("Lord Omar Khayyam Ravenhurst"), present the worship of Eris, goddess of discord, whom classical myth blamed for starting the Trojan War by rolling a golden apple marked 'to the fairest' among the goddesses.
The doctrine, so far as there is one, is that order and disorder are both illusions laid over reality by the human nervous system, and that the Discordian, by embracing chaos, sees through the imposition. It comes with a mythology held with visible irony: the Law of Fives (that everything is related to the number five, if you look hard enough), the Sacred Chao with its Hodge and Podge, the word fnord, and a calendar of five seasons. Its best-known line is the question of whether Discordianism is "a joke disguised as a religion, or a religion disguised as a joke", which it pointedly refuses to answer.
Two things keep it from being merely a prank. The first is its reach: through Robert Anton Wilson and Robert Shea's Illuminatus! trilogy of 1975, which drew heavily on it, Discordian ideas and jokes, fnord above all, passed into a much wider counterculture and, later, into internet culture. The second is that scholars of religion take it seriously as an object of study. Carole Cusack and others treat it as the earliest fully developed 'invented religion', one that knows it was made up and treats that knowledge as the point, and argue that the line between such a thing and a religion of the ordinary kind is harder to draw than it looks. Dated honestly, it is a 1963 American satire that became, for some of its participants, a working spiritual practice without ever ceasing to be a satire.