The truth, the whole truth and nothing but the truth (except for the stuff that is total Bull (which is most of it, probably))
Sci fi, fantasy and horror are three "thriller" genres that are often lumped together for very good reason. The dividing line between them can be so thin that it's hardly there.
For example: Gulliver’s travels is a political satire guised as a fantasy. In it, Gulliver travels by boat to strange new worlds where he meets giants and tiny people and talking horses. Had it been a starship to a foreign moon instead of a sea ship to another land, the story would have been science fiction.
The chief distinguishing characteristic is that sci-fi usually involves space travel, space aliens, or time travel...and can we please have some smart-ass robots? Thank you!.
Star Wars from 1977 is a wonderful blend of Sci-Fi and Fantasy. A long time ago (fantasy) in a galaxy far, far, away (science fiction). The film had robots and spaceships and warp speed and interplanetary travel: all tropes of science fiction. But at its core was a tale of chivalry, of rogues and heroes, knights and princesses; a tale full of fantastic creatures and magic spells.
Frankenstein is a gothic horror, but the reanimation theme is much closer to sci-fi. And Isn't HAL from 2001: a Space odyssey, a villain as dangerous and cool as Hannibal Lecter? What is Darth Vader but a Frankenstein of the future?
At the heart of the best of all three genres is a morality play about the human condition. Lord of the Rings is a fantasy, yet, had Tolkien placed Middle Earth on the surface of Mars, it would be called sci-fi.
To make it simple: we usually associate sci-fi with space travel, and the most important element of that is aliens. A second common element is robots/replicants/artificial intelligence. A third common theme is that it takes place in the future. And lastly, the weirdness is usually explained, or we can draw conclusions about what is going on. (Having used ATMs and seen animatronics in Disney, the Johnny-Cab in Total Recall is an element of sci-fi; we can understand how such a thing could come to be.)
This is different is from the walking trees in the Lord of the Rings, or the magic spells in Harry Potter. We are supposed to watch the Terminator with the idea that what we see on the screen is possible in the future. If we saw a sci-fi movie with a planet of walking trees, it would still be sci-fi because of the interplanetary travel and the Aliens, but as with star wars, there would be a strong fantasy element.
When I mentioned Horror as part of this genre, I was of course referring to the classic horror from "Nosferatu" to "Night of the living dead". The modern horror of "Saw" is something else. I would also exclude slasher films like "Friday the 13th, "Halloween" and even classics like "Psycho". A crazy person who kills innocents is horrifying and horrible, but unlike classic horror and sci-fi, there is no element of the fantastic in it.
I guess the bottom line is an element of the supernatural. Poltergeist may qualify. Jack the Ripper feels like he ought to be made an exception because of his gothic locale and his never having been caught. He seems supernatural in a way that fictional monsters like Michael Myers from Halloween do not.
The Blob is a horror that is sci fi, a nuclear blast or a space spore always feels like sci-fi, and the less anthropomorphic a creature it is the more “alien” it feels.
The Mummy, Dracula, Frankenstein and the Wolfman feel like creatures from the past (and in some cases are). The Creature from the Black Lagoon may be more primordial than the others but he has a sci-fi space alien feel to him, doesn’t he? The same with Zombies.
Sometimes the difference is whether the evil is unleashed by an ancient curse or by cosmic radiation.