While reading another questions, I had to think about what is the relationship between fantasy and science fiction? Is science fiction just a sub-genre of fantasy? If they are considered different genres, what are the key characteristics that define them and keep them from being one genre and a sub-genre?
The essential difference is that science fiction is supposed to be plausible at the time - it requires little or no suspension of disbelief. It is generally (but not always) set in the future, and involves advanced technology which allows certain breaks from current limits.
Fantasy is traditionally set in the past, or in an alternate universe which resembles certain times in our past. It generally requires more suspension of disbelief - magic and the supernatural play a much larger part.
These aren't hard and fast rules, of course.
The technical definition of Science Fiction is as a subset of Fantasy.
Fantasy in common use, however, has become a term for settings that (any one or more of):
- lack modern technology
- have magic
- make use of improbable† pasts as setting tropes (as with Conan and Kull)
- have settings with altered physics‡
Generally, Science Fiction has come to mean settings that (any one or more of):
- set in space or across space
- have higher technology than modern
- have a clear recent divergence point and alternate history
- have one clear break in physics‡
- have aliens
- FTL travel
Sci-fi may include
- limited magic in the form of psionics, especially telepathy and clairavoyance.
- alternate timelines based upon clear divergence points/events.
Several settings are sci-fi, but have fantasy trappings, including McCaffrey's Pern setting... it's set on a world around the star Rukbat. Ironwood, by Bill Willingham, is a connected alternate universe, with a strogn fantasy feel, and imported Marlboro cigarettes...
† most would say impossible pasts, now, as almost no one seriously believes in prediluvian cultures any more
‡ I've read several short stories where the change was a change in physics and exploration of it. In fantasy, usually as means of having medieval settings without gunpowder, as the universe differs by gunpowder not generating a detonation. In science-fiction, it has been used as a means of exploring other elements of culture. In both, it straddles the borderline. Often called Science Fantasy.
Simple and short, as stated by Prof. Farnsworth (or rather his alter ego The Great Wizard Greyfarn)
[..] instead of science, we believe in crazy hocus-pocus.
(from Bender's Game)