Science Fiction is a the name given to a broad category of fiction that deals with imaginary, but more or less plausible, worlds, and to some degree embraces scientific ideas used in a generally speculative fashion. Such works may be set in the future or in alternate timelines. They may be set in outer space, or on other planets. They may include extraterrestrials, robots, or other non-human characters. Characters in such stories often use "futuristic" technology that, while unavailable in the present, could conceivably exist at some future time.
Authors and readers alike have varied definitions of what constitutes science fiction, and a number of sub-genres have developed. Alternate history is often described as a sub-genre of science fiction, although it may also be described as historical fiction or literary fiction. Fantasy is technically different from science fiction, although many literary works cross over elements of both genres into the same story.
Harry Turtledove has written a number of science fiction tales, beyond the alternate history he is arguably most famous for. Some works of his alternate history have an undercurrent of science fiction, most notably The Guns of the South and Worldwar.