I’ve been asked this question often since I started this site, and I’ve only been able to give a very vague definition- usually I say, “speculative fiction” is a kind of umbrella under which we can find sci-fi, fantasy, horror, magical realism, etc. But what are these (sub)genres, actually? Of course, some of the best writing crosses these boundaries, but I myself find it helpful to have some handy general definitions around when I’m wondering, for instance “is this story more ‘weird’ or ‘magic realism’? Or a combination?” We humans generally like classifications/categories, and they can be useful in discussions of themes and concerns in speculative fiction.
I’m drawing many of these definitions from the excellent Science Fiction Encyclopedia.
soft science fiction: used to identify sf that deals with the “soft sciences” (social sciences and related fields) or to sf that doesn’t deal with recognizable science at all, but emphasizes human feelings; often more concerned with character and speculative societies, rather than speculative science or engineering. Examples: The Time Machine, The Martian Chronicles, The Left Hand of Darkness.
hard science fiction: summarized by Allen Steele (in “Hard Again” in New York Review of Science Fiction, 1992): “Hard sf is the form of imaginative literature that uses either established or carefully extrapolated science as its backbone.” Examples: Solaris, Primary Inversion, The Three-Body Problem.
fantasy: stories that include one or more of the following: Gothic motifs, dragons, gods and demons, golem, magic, monsters, mythology and supernatural Creatures. Examples: The Lord of the Rings trilogy, the Discworld series, Fledgling.
magic realism: originally described a kind of literature associated with 20th century Latin America; according to Alejo Carpentier, MR is “an apprehension of the whole of reality, rather than of a particular (already outmoded, and confinedly anglophone) fictional genre…in order to comprehend the world, the world must be addressed as fabulous, without any implication that the fabulousness of the world was in any sense extrinsic.” Examples: One Hundred Years of Solitude, 1Q84, The Island of Eternal Love.
(new) weird: coined by China Miéville in a guest editorial for The Third Alternative (Summer 2003); “Complex, grimy urban-noir settings and a fondness for grotesquerie are characteristic but not necessarily defining qualities. Perhaps more useful is the sense that New Weird stories freely mingle sf, dark fantasy and horror tropes….” Examples: Annihilation, Embassytown, The Core of the Sun.
horror: “horror can deal with the mundane or the supernatural, with the fantastic or the normal. It doesn’t have to be full of ghosts, ghouls, and things to go bump in the night. Its only true requirement is that it elicit an emotional reaction that includes some aspect of fear or dread.” (Horror Writers Association). Examples: Black Tea and Other Tales, Hex, It.
cyberpunk: “cyber” relates to cybernetics: to a future where industrial and political blocs may be global rather than national, and controlled through information networks; a future in which machine augmentations of the human body are commonplace, as are mind and body changes brought about by drugs and biological engineering. Central to cyberpunk fictions is the concept of virtual reality; “punk” comes from the rock’n’roll terminology of the 1970s, referring to that which is young, streetwise, aggressive, alienated, and offensive to the Establishment. Example: Neuromancer, Cryptonomicon, The Stars My Destination.
steampunk: coined by K W Jeter in a letter (April 1987 Locus) – by analogy with cyberpunk – to describe the modern subgenre whose sf events take place against a 19th century background; often refers to works set in an era or world wherein steam power is still widely used—often set in Victorian era England—but with prominent elements of either science fiction or fantasy, such as fictional technological inventions (see H. G. Wells and Jules Verne), or real technological developments like the computer occurring at an earlier date. Examples: The Best of Spanish Steampunk, Perdido Street Station, Infernal Devices.