Ray Bradbury had this to say about science fiction and fantasy, as quoted from the introduction to (I believe) The Circus of Dr. Lao and Other Improbable Stories:
Science-fiction is the law-abiding citizen of imaginative literature, obeying the rules, be they physical, social, or psychological, keeping regular hours, eating punctual meals; predictable, certain, sure.
Fantasy, on the other hand, is criminal. Each fantasy assaults and breaks a particular law; the crime being hidden by the author’s felicitous thought and style which cover the body before blood is seen.
Science-fiction works hand-in-glove with the universe.
Fantasy cracks it down the middle, turns it wrong-side-out, dissolves it to invisibility, walks men through its walls, and fetches incredible circuses to town with sea-serpent, medusa, and chimera displacing zebra, ape, and armadillo.
Science-fiction balances you on the cliff. Fantasy shoves you off.