Fandom is a noun born of a combination of fan with the affix -dom (i.e., kingdom). The affix implies that fandom is a (non-physical [though fandom can and does have physical manifestations in art, conventions, etc.]) realm for the fans of a given media work.
The fandom is generally an extension to the universe explored and created in the original book(s), television shows, and/or film(s).
Fandom and Web 2.0
Most fandoms were early adopters of the ease in communication offered by Web 2.0. Many have built thriving online communities, particularly with the aid of forums.