A category of literature.
A genre can categorize literature by attributes such as writing style, content, themes, intent, or length. Because of this, genres often overlap, and multiple genre labels may be used to describe the same work.
(Some people use the term genre only for labels which describe tone and content, using other terms for labels which describe other attributes, for example length.)
Most genre labels are loose categories which shift over time, and firm definitions are rare.
Many genres are further divided into sub-genres, which inherit the characteristics of the larger genre and add further specification.
The genre labels of fiction and non-fiction are based on the factuality of the content. (Normally, any book which is not wholly factual is classified as fiction.)
Examples of fiction genres based on length include short story, novelette, novella, novel, and epic novel.
Examples of fiction sub-genres based on content include (but are by no means whatsoever limited to) adventure, comedy, satire, erotic, historical fiction, thriller, science fiction, fantasy, horror, mystery, and romance.
Also see: fiction, non-fiction, sub-genre, short story, novel, age label.