A small group of people on a forum who are disruptive, rude and annoying to the other members. Most often, there are two or more members of the forum clique who are "posting pals" and reply to each other ad nauseam. Then, their followers feel they have enough balls to get in on the action and gang up on newer or weaker members. People who don't agree with their views are flamed, harassed and most often driven out, usually at the expense of the site itself. This behavior is seen on forums with little to no moderation or where the mods themselves are part of the clique. There's usually a spineless or non-caring administrator who allows this behavior.
Person 1: Do you ever go to that one website?
Person 2: The one with the terrible forum clique that makes it a shithole?
Person 1: Yep, it sucks, just like the cliquey douchebags who post there.