There are no universally-agreed-upon definitions of science fiction, fantasy and horror, because they often overlap. As Arthur C. Clarke once said, "Any sufficiently advanced technology is indistinguishable from magic." These are the working definitions used for categorizing films in this database, and overlaps are dealt with simply by including a film in multiple categories.
Science Fiction: Any story that includes technology significantly advanced from our own, future settings, alien creatures or worlds, or alternate histories.
Fantasy: Any story that includes magical people (wizards, witches), magical creatures (dragons, unicorns), mythological details (gods, fairies), or intelligent animals (unless the intelligence was the result of science).
Horror: Any story intended to convey a feeling of fear or dread. Horror stories often include elements of the supernatural (ghosts, spirits, demons), undead (vampires), or people, animals or objects behaving in unnatural ways.