A genre is a division of a particular form of art or utterance according to criteria particular to that form. In all art forms, genres are vague categories with no fixed boundaries. Genres are formed by sets of conventions, and many works cross into multiple genres by way of borrowing and recombining these conventions. The scope of the word "genre" is usually confined to art and culture. In genre studies the concept of genre is often compared to originality.
Subgenre
Genres are often divided into subgenres. Literature, for instance, can be organized according to the "poetic genres" and the "prose genres". Poetry might be subdivided into epic, lyric, and dramatic, while prose might be subdivided into fiction and non-fiction. Further subdivisions of dramatic poetry, for instance, might include comedy, tragedy, melodrama, and so forth. This parsing into subgenres can continue: "comedy" has its own genres, for example, including farce, comedy of manners, burlesque, and satire.
Science Fiction has perhaps more generally recognized subgenres than many other fields of literature as a science fiction story may be firmly rooted in real scientific possibilities (See: Hard science fiction) as they are understood at the time of writing, or be highly and speculatively imaginative tales set in an extraterrestrial civilization for example, or in a parallel universe, an Alternate history, or outright Fantasy, all recognized subgenres of science fiction. A perhaps more apt term as coined by Heinlein all part of, 'Speculative fiction' Thus, even fiction that depicts innovations ruled out by current scientific theory, such as stories about or based on faster-than-light travel, may still be classified as science fiction. And more recently, the term "science fantasy" which category covers stories which have elements of both hard science and fantasy has come into play as a subgenre.