Fandom is a general term for the science fiction fan community. Members of fandom often refer to themselves as fen, thus distinguishing themselves from fans of the genre who are not part of the fannish community. The main difference being their active involvement in fandom itself.
"Fandom is a very worthwhile hobby, and the most worthwhile thing in it is doing as well as you can something that interests you and gives other people pleasure, no matter how much trouble it is." -- Walt Willis, Warhoon 28