The Difference Between Science Fiction and Fantasy
Over at Yahoo! Answers, someone asked "What is the difference between science fiction and fantasy?," and I decided to post my own answer to that common question:
Science fiction deals with things that might possibly happen (or, in the case of the subgenre of science fiction known as alternate history, things that possibly could have happened); fantasy deals with things that never could happen.
There is always a path from our here-and-now to the milieu of a science-fiction story: usually that path simply involves time passing and plausible advances in science and changes in society taking place during that time.
There is never a path from our here-and-now to the milieu of a fantasy story: no matter how much you might want to get to the fantasy world, you can't, because magic and supernatural powers do not work in our universe -- you can't get there from here.
Succinctly: there's discontinuity between our reality and fantasy; there's continuity between our reality and science fiction.