First off, if you've somehow managed to find yourself on a website called Discordian.com, you probably already know what Discordianism is. While each person's perception of Discordianism differs, if you've gotten here at all, you probably have a vague idea of what it means to you. However, for those of you who don't know what Discordianism is, the Jargon File defines Discordianism as:

Discordianism /dis-kor'di-*n-ism/ n.

The veneration of Eris, a.k.a. Discordia; widely popular among hackers. Discordianism was popularized by Robert Shea and Robert Anton Wilson's novel "Illuminatus!" as a sort of self-subverting Dada-Zen for Westerners -- it should on no account be taken seriously but is far more serious than most jokes. Consider, for example, the Fifth Commandment of the Pentabarf, from "Principia Discordia": "A Discordian is Prohibited of Believing What he Reads." Discordianism is usually connected with an elaborate conspiracy theory/joke involving millennia-long warfare between the anarcho-surrealist partisans of Eris and a malevolent, authoritarian secret society called the Illuminati.

There is, of course, a whole lot more to it than that. Discordian.com deals more with the question of "So I'm Discordian - now what?" For some more in-depth definitions of what Discordianism is to begin with, see these sites:

The H2G2 entry on Discordianism
Disinfo.net's dossier on Discordianism: A Religion Disguised As A Joke Disguised As A Religion
The Alfredian Discordian's page on Discordian History
The Encyclopedia of Religious Movements' description of Discordianism