Fandom (from the noun fan and the affix -dom, as in kingdom, dukedom, etc.) refers to a community of keen aficionados who share a common interest in any phenomenon, such as authors, hobbies, genres or fashions. Fandom as a term can also be use to refer to the single interconnected network of these individual fandoms, many of which overlap.
Fans (or the plural fen) typically are interested in even minor details of the object of their fandom; this is what differentiates them from those with only casual interest.
The objects of a fandom typically relate to the arts, sports or entertainment. For example, it would be unusual to refer to an accountant who is very interested in the details of accounting as a “fan” of accounting.
Members of a fandom associate with one another, often attending fan conventions (such as science fiction conventions), and publishing and exchanging fanzines. Today, these communities are often online, especially for less well-known source material.
Some fans also write fan fiction, stories based around the universe and characters of their chosen fandom. Some also dress in costumes (“cosplay”) or recite lines of dialogue either out-of-context or as part of a group reenactment.
The term “fandom” is particularly associated with fans of the science fiction and fantasy genres, a community that dates back to the 1930s and has held the World Science Fiction Convention since 1939. The Oxford English Dictionary traces the usage of the term back as far as 1903, with many of its documented references referring to sports fandom.
The term is also commonly associated with anime/manga. Serious fans of this subject are also called otaku.
“Fandom” is also the name of a documentary / mockumentary about a fan obsessed with Natalie Portman. (via Wikipedia)
Sexy Fandom on Twitter 
