Fandom (from the noun fan and the affix -dom, as in kingdom, dukedom, etc.) is a subculture composed by like-minded fans characterized by a feeling of closeness to others who share the same interest. There are Fandoms for almost every Movie, Book and TV show you can find. Fans of these come together to discuss, comment and theorize over their Fandoms topic on a variety of web sites.
For some series, such as Harry Potter and Star Wars, which has a large fan base toping an others, fans are constantly looking for clues as to what is to come next, taking the stories apart and searching through every minor detail. In large Fandoms like these, there are usually many clashing opinons and arguments (see Ships)
Though members of a fandom do a lot more than discuss with each other on the Internet. It can branch out into a wide variety of things, such as Fan Fiction, Fan Art.