Fanfiction
Fanfiction is fiction written by people and based on a certain work by an original writer, be it film, book or television etc. It makes use of the characters and situations in the work, further developing them through the imagination and words of the fanfiction author.Fanfiction spans many fandoms and comprises many, many, many types. There's crossovers, alternate universe, real person fic, songfic, poetry, slash, and so much more... and of course the much-detested Mary-sue. For a thorough coverage of all these, proceed over to Wikipedia.
Genres
Fanfiction can be divided into different genres. They are as follows:Gen: short for "general" - as the name implies, gen fanfiction is suitable for anyone of any age. It covers a wide range of topics and contains no sexual overtones. Generally rated G.
Het: short for "heterosexual" - fanfiction containing romantic or sexual relationships between a opposite genders. Ratings can range from G all the way to NC-17.
Slash: fanfiction involving a romantic or sexual relationship between characters if the same gender (i.e. male/male or female/female). Mostly used for m/m pairings; f/f stories are often called femmeslash. Ratings are usually PG to NC-17 though most stories are rather unsuitable for children...
Adult: I think this is self-explanatory. Stories rated Adult are unsuitable for childred due to excessive sexual content, violence or mature subject matter.