Science Fiction : A popular modern branch of prose fiction that explores the probable consequences of some improbable or impossible transformation of the basic conditions of human (or intelligent non‐human) existence.
Fantasy : A general term for any kind of fictional work that is not primarily devoted to realistic representation of the known world. The category includes several literary genres (e.g. dream vision, fable, fairy tale, romance, science fiction) describing imagined worlds in which magical powers and other impossibilities are accepted.
Source: The Oxford Dictionary of Literary Terms. Chris Baldick. Oxford University Press, 2008.