1984 is a work of dystopian fiction, which itself belongs to the broader category of speculative fiction. Speculative fiction is an umbrella of genres that imagine alternate worlds, histories, or futures, and thereby encompasses genres like science fiction, fantasy, and magical realism. While some might be tempted to call 1984 a work of science fiction, this label would not be entirely appropriate, as the laws of science in the world of Big Brother seem to correspond with the laws of science in the real world. They certainly possess technology in Oceania that did not yet exist in the 1940s, but this is not enough to qualify the novel as science fiction. Rather than creating a world with its own scientific laws for the novel, Orwell speculates about what happens when totalitarian repression roots out freedom of thought.
Central to Orwell's speculative project is his creation of a hypothetical history. In order to imagine a dystopian future, he must also construct a partially dystopian past. Many elements of his fictional world are recognizable to the reader, such as some of the local geography, world geography, and various historical references. London exists in the world of the novel, and the rise of both Nazism and communism shaped the history of that world. Many other elements differ, however. It is unclear whether the world wars took place in the same way, and the continents seem to be divided up differently (into three superstates).
What makes Orwell's novel dystopian is that it is set in a society where history has culminated in highly undesirable outcomes. Governed by the repressive Party regime and structured by the terrible policies of Ingsoc, this society is plagued by strict surveillance, all-consuming censorship, and violent repression.