I guess media fandom in general occurs when people not only become knowledgeable about & deeply personally invested in a story, but when they do it in a group (even if it's only to meet rarely at cons, or to talk about it online.)

I'm involved in the Phantom of the Opera fandom, and to a lesser degree LOST (i.e. I follow it regularly and read/participate in a big LOST forum.) Also, have written fanfiction in both, although far more in POTO.

Well, trying to be nice re: terms. Sometimes fans can get a bit ... fanatical? i.e. not stopping to catch a breath & realize that not everyone sees a story, characters, etc. the way you do. (Am thinking of violent passions aroused by "ship wars," for instance.)