Someone who is obsessed with a particular tv show, book series, video game, or other mainstream entertainment -- typically of the sci-fi/fantasy genre -- to the point where they will collect every marketable product available related to the SBG (show/book/game) which they can afford. They will defend their obsession vigorously, even violently, against fans of other SBG's, or even against fans of the same SBG who ask them to even consider the remote possibility that the SBG has any kind of flaw whatsoever. Frequently seen in chat rooms or Internet message boards posting in incomplete sentences and swearing a lot.

Fangirls have been theorized to exist, but have never been seen in the wild.

A slang term for, usually, a rabid fan of comic books. Stereotypically, fanboys are the guys you see who wear comic book T-shirts, spend every last dime they have on new comics, are capable of arguing the finer points of comic book continuity for hours on end, and have more important things to do than bathe. The truth, of course, is much more complex: most of us shower every day.

I've sometimes heard the term used to describe everyone from roleplaying game fans to Trekkies to Linux geeks, but the default value seems to be comic book nuts...