It seems like sci-fi always take place either in the future, in space, and/or on another planet. Because it has to do with hypothesis of what the future or life on other planets might look like.

The main one? Science fiction contains scientific elements, recognisable to our own (i.e. Spacecraft, electrical appliances etc). Fantasy is more that which is unrecognisable, or very different from our own world - 'Lord of the Rings', for instance. A blend between the two is 'Star Wars' - gadgets and items that are recognisable in a galaxy very different from our own.

Sci-fi always has a connection to reality and science in some way. The devices, however imaginative, are based on the world we have. Fantasy has no such restrictions. An example is that in fantasy you use a spell to go backwards in time. In Sci-fi you use a time machine.

Sci-fi is something that's reasonably possible, where fantasy has no real limits.

A very good question. This can and has been debated endlessly by fans of the genres. The simple answer is that SF involves scientific or technological elements, fantasy does not. However, this is an unsatisfactory answer for a number of reasons. Modern urban fantasy clearly includes a certain amount of technology, but is Harry Dresden SF? Of course not. What about Star Wars or other similar works, where the science and technology are played fast and loose and magical powers (the Force) work? One might define Fantasy by the inclusion of magic, but what of psionic powers, assumed to have a rational scientific explanation? The lines blur a lot sometimes, nobody seriously suggests Lord of the Rings is SF, but there are those who will argue that Star Trek is fantasy. It's a never ending, never to be resolved debate. http://bookclubs.barnesandnoble.com/bn/board/message?board.id=fsf&thread.id=13&jump=true http://www.treitel.org/Richard/sf/fantasy.html http://www.sfsignal.com/archives/2005/12/world-building-fantasy-vs-science-fiction/ http://www.scalzi.com/whatever/003887.html

http://www.watt-evans.com/sfvsfantasy.html

Short form answer? Science fiction typically deals with the implications of some advance in science or technology that is at least arguably plausible in the future. Fantasy typically involves magic and the paranormal -- in other words it is usually set in worlds where the laws of physics operate differently, and often where those different physical laws have been known for a long time before the story starts.

Ok, I'm finding all the other answers a bit lacking. I am a huge geek, so I've discussed this at length with friends (and husband). Sci-fi uses conventional science to create unrealistic but probable worlds which rely on the use of technology to enhance the story, but not tell it. It is also called speculative fiction. Mostly the authors play what-ifs with technology, using science as we see it now as a basis, but creating a world by pushing that to its furthest. It uses what we know to be true already. I think this is what makes the foundation series so creepy. It could happen... if! The line is, if its a western, it ain't sci-fi. (And firefly doesn't count here) Meaning just because something has superior technology in its storyline doesn't mean its really sci-fi. But I'm a bit of a purist. Fantasy on the other hand is concerned with the same things- creating a world but rather than using science or technology they use magic. It uses what we know to be false in order to make us more accepting of the story. This has pretty much been said, but the thing that tends to be confusing is that when science is pushed so far as it is in sci-fi, it becomes magic anyway. Clarks law- http://en.wikipedia.org/wiki/Clarke%27s_three_laws Theres also quite a few sub-genres that distinguish stories that have elements of each. For example: I consider Star Wars to be neither sci-fi or fantasy, but a space opera because it not only has both technology and magic, but it uses such clear archetypal characters and that giantly grand scope. That was kinda long. I may take this a bit too seriously... sorry people.

Scifi is like finding life on other planets, but fantasy is that there is a woman with eight vaginas, or something.

Fantasy has skimpier costumes and prettier planets.

I'd say just look at the genres. Sci-Fi Star Trek Star Wars Battlestar Galactica Maybe....something like 28 Days Later? It's horror but also a bit science fiction. Science Fiction.....fiction.....on science....Fake Science. Fantasy Lord of the Rings Trilogy Legend Labryinth Narnia Fantasy is what I'd say more concerned with stuff that really has no explanation, unicorns, wizards, dragons, stuff like that.