Fans are people into one thing, person, or hobby. Using our friendly online etymology dictionary we can learn that “fan” most likely comes from fantatic, itself “pertaining to a temple” or “fanum.” So a fan is someone who is dedicated to something in an almost religious sense. In Japanese there is the similar term “Otaku,” which, as wikipedia describes it, “is a Japanese term used to refer to people with obsessive interests, particularly anime, manga, and video games.” This specific emphasis on technology is similar to the slang term, “nerd,” a relatively new word in American English that refers to someone with an obsessive relationship with knowledge or originally a conservative traditionalist.
So fandom always implies two things, the original and the place from which it is appreciated. A fan of the anime from Kyoto is similar to but different from a fan of anime from Orange County California.
Nerd, otaku, and fan are all now positively used. They are a marker of inclusion in a community of interest and a sign that one is passionate about something. The valorization of nerdiness is a trend against the image of cool, e.g. a kind of laid-back disinterest. This can be overstated but it it interesting to think if a larger trend toward nerd or fan culture is a reaction against the individualism of a lone rebel. Surely on some level fan cultures and communities sometimes form tighter bonds than traditional neighborhood and geographic definitions of community.
A few documentaries that deal with Fan Cultures and Nerd or Otaku Culture:
Heavy Metal Parking Lot
Fast, Cheap, and Out of Control