In its broadest sense, "Fantasy Fiction" covers an immense number of works by many authors, from ancient myths and legends, to some recent works embraced by mainstream literary audiences (such as Neil Gaiman's best-selling novel American Gods) and much in-between.
As a genre, fantasy is both associated and contrasted with science fiction and horror fiction. All three genres feature elements of the fantastic, of making radical departures from reality or radical speculations about what reality might be like, or might have been like. Some writers and critics prefer the term Speculative fiction due to the frequent crossover from one genre to another.
Further blurring the definition, some suggest there is a distinction between "Fantasy" proper as a genre, and "the fantastic," the latter being a fantasy-like element in other fiction.
Comic fantasy -- especially the works of Terry Pratchett -- should also be mentioned here, which parodies the above ideas as well as ideas outside the genre, in a postmodern manner. This category might also include the so-called 'worst science fiction story ever published' The Eye of Argon.
Role playing games in turn spawned more fiction in the genre. Game companies have published fantasy novels set in their own fictional game universes; the Forgotten Realms and Battletech series are some of the more popular.
Similarly, series of novels based on fantasy films and TV series have found their own niche.