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 consistutes 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.
Harry Turtledove has writen a number of science fiction tales, beyond the alternate history is arguably most famous for.