Fandom
Keen aficionados of any phenomenon such as authors, hobbies, ideologies, genres or fashions can collectively manifest as fandom. 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 fandom are typically artistic, sporting or entertainment related. 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. Fandom members often attend various fan conventions, like the science fiction convention.
"Fandom" can also refer to the community of fans (usually online) who all share a fandom. These communities are often online, especially for less well known source material, but can also exist in the real world. "That forum is the biggest in the fandom"
A commonly used pejorative term for a member of fandom is "fanboy" or "fangirl".
The term "fandom" is particularly associated with fans of the science fiction and fantasy genres.
Many fans also write fan fiction, stories based around the universe and characters of their chosen fandom.
There are also negative elements associated with fandom, with some seeing these fans as being overly enthusiastic to the point of fixation or unhealthiness, or to the point where it negatively impacts their relationships with other human beings. Many fans cope with this by only associating with others in their respective fandom.
Some fans are also seemingly unafraid to engage in activities considered embarrassing, such as dressing in bizarre costumes ("cosplay") or reciting lines of dialogue either out-of-context or as part of a group reenactment. Many of these particular fans retort that doing such things is simply a method of appreciation or expression, and part of an otherwise healthy life which does no harm to others. Others have said that perhaps these eccentric activities, while not common, are healthier than other outlets available for those who do not exactly fit within the "normal" world.