This was a post that I put up over on the romance thread, but it's equally applicable here.
Currently, on the shelves, there are a number of different things that wind up shelved in "Science Fiction/Fantasy". But there are definite differences. These are the things that I've gleaned from talking with various editors and authors in the genres:
I write paranormal, and it's part of a much bigger grouping, called FFP (Futuristic, Fantasy & Paranormal,) or SFF (Science Fiction, Fantasy, Futuristic) or just plain paranormal. Many people wonder what an "otherworldly" book should be called, so here are some guidelines:
Futuristic: A futuristic novel is one that is from 2006-infinity, but is EARTH-BASED! This means that you are using humans as they exist on Earth that may or may not interact with other species that have yet to be discovered, or dealing with a world very different than what we now know (whether from natural catastrophe, alien invasion, world politics, etc., etc.) The future world must have rationale that is understandable. For example, humans don't have green blood. They will probably NEVER have green blood, so don't try it. But Vulcans (Star Trek) DO have green blood, so that's okay. Star Trek is a tricky one, because it IS science-fiction, and it IS futuristic, and big chunks of it ARE fantasy. It's generally considered science-fiction, for the record.
Fantasy: The biggest issue with the fantasy sub-genre is the concept of "world-building." Fantasy novels come in two breeds: One is a different world, with creatures that don't exist on earth, that may or may not talk, etc. The other fantasy is "alternate reality." An alternate reality is one which follows Earth history except for one or two things. The Laurell K. Hamilton, Anita Blake world, for example is a good example. Vampires have always existed. But they were hunted like rats -- UNTIL the United States Supreme Court declared them "not dead". Suddenly, vampires could own property, adopt children (since they couldn't bear them on their own), marry, divorce, etc. Probates for "dead" relatives were unwound and life generally was upended. The key to a fantasy is making the fantastical elements an equal partner to the regular plot. So the guy is a vampire -- some are jerks and some are sweeties. It's an ELEMENT of the personality, but they don't have to fall into "set" requirements, because it's NOT REAL, so it doesn't have to adhere to known legends. And yes, there is almost always magic of some form involved.
Science Fiction: This is often confused with futuristic and fantasy, but the goal of science fiction is the USE OF science in the story. Whether the story is set in 2005 or 3035, hard science that is well thought out is key. The Time Machine or 20,000 Leagues Under the Sea are science fiction, but not necessarily futuristic or fantasy.
Time Travel: Also a close contender for futuristic. After a long and... ahem, spirited discussion among the authors of a writer's loop I belong to, we determined that the difference between futuristic and time travel is one key element. Susan Grant's The Scarlet Empress was the issue at hand -- should it be entered in a contest under time travel or under futuristic. The joint decision was made that it was futuristic, because THE HEROINE COULD NOT RETURN HOME. Susan's heroine was cryogenically frozen and re-awoken in 2176 to save the world from a "new world order". It was science fiction, and futuristic and was a woman from the past thrown into the future. But we finally all agreed that for time travel to be a key element in the novel, the hero or heroine must have the ABILITY to return to their time, whether or not they choose to do so. Otherwise, it's in a different category.
Paranormal: The key element of a paranormal is LEGEND. Werewolves are legend, and so are vampires, pixies, fairies, doxies, living mummies, etc. A paranormal is PARA or "resembling or imitating" NORMAL "real life". Most vampire novels fit this category, although they might be shelved in horror. There is a handed down legend or fable or "this really happened!" account of something outside the ordinary that we don't generally believe but aren't complete convinced COULDN'T happen. While it can be set in the future, the past or the present, the key is that everything ELSE is normal, except for the addition of this one element.
Does that help any? Or just give you more questions to ask? LOL!