While science fiction was once primarily "about science," it has since evolved into a far-reaching field that encompasses a variety of subgenres. When pitching your novel or sending your short story to a specific market, it's helpful to know just where it "fits" in the field; this guide should help.
Alternate history asks, "What if a historic event had turned out differently?" Harry Turtledove is a master of this subgenre; his Guns of the South asks what would have happened if the South had won the Civil War, while his World War II: In the Balance series blends alternate history with "first contact" by asking what would happen if aliens had invaded Earth during WWII.
Apocalyptic, holocaust, and post-apocalyptic stories focus on the end of the world, or the world just after "the end." In Niven and Pournelle's Lucifer's Hammer, society is wiped out by a comet and must rebuild; in Nevil Shute's On the Beach, humanity is destroyed by a nuclear holocaust; in Stephen King's The Stand, humanity must cope with the aftermath of a devastating plague.
Cross-genre stories defy easy distinctions between science fiction and other genres, such as fantasy ("if it's psychic power, it's science fiction; if it's magic, it's fantasy"). Christopher Stasheff's Warlock in Spite of Himself series, for example, places a space-traveling agent on a planet apparently populated by witches, werewolves, and other fantasy beings. Such novels may also blend science fiction and romance, mystery, suspense, and even Westerns (as in the Brisco County television series).
Cyberpunk is set in a high-tech, often bleak, mechanistic and futuristic universe of computers, hackers, and computer/human hybrids. The subgenre was pioneered by Bruce Sterling and William Gibson, who coined the term "cyberspace" in Neuromancer. Humans may have built-in computer jacks or software ("wetware"), and spend considerable time "living" in a virtual environment, as in The Matrix.
First contact explores the initial meeting between humans and aliens, ranging from horrific tales of invasions to stories of benign visitors bearing the secrets of advanced technologies and world peace (or irony, as in The Twilight Zone episode "To Serve Man" -- the one that ended, "It's a cookbook!"). The meeting may occur on Earth, in space, or on another planet. H.G. Wells' War of the Worlds helped define the "alien invasion" variant of this subgenre.
Hard science fiction is driven more by ideas than characterization. Plausible science and technology are central to the plot. If your story is set on a lunar colony, for example, issues of technology may be of greater concern than a character's personal life. To write effectively in this subgenre, an author must generally have a good grasp of the scientific principles involved. Much classic science fiction, including the earlier works of Asimov and Heinlein, fall into this category.
Light/humorous science fiction may occur within any of these subgenres, or (often) spoof a subgenre. Douglas Adams' Hitchhiker's Guide to the Galaxy is one of the best-known examples of humorous science fiction.
Military science fiction looks at combat in future locations (space, another planet), against a range of opponents (modified humans, aliens, machines), with futuristic, high-tech weaponry (including genetically modified soldiers). While some military science fiction asks "how fast can we blast the bugs?" (Robert Heinlein's Starship Troopers), many authors use this subgenre to address anti-war themes. David Drake's Hammer's Slammers series explores both the heroism and the carnage of warfare.
Near-future science fiction takes place in the present day or in the next few decades. Elements of the setting should be familiar to the reader, and the technology may be current or in development. Stories about nanotechnology or genetics, such as Greg Bear's Blood Music, often fall into this category.
Science fantasy/future fantasy, rare now but popular in the 1930's and 1940's, alters, breaks, or ignores known laws or scientific theories for the sake of the story. Edgar Rice Burroughs' Barsoom novels (set on Mars) are a good example.
Slipstream deals with "mainstream" themes but contains a speculative element. Margaret Atwood's The Handmaid's Tale is set in a speculative future, for example, but is marketed as a mainstream novel.
Soft/sociological science fiction is character-driven, with emphasis on social change, personal psychology and interactions, etc. While technology may play a role, the emphasis is not so much on how that technology works, but how it affects individuals or social groups. Robert Silverberg's short story "To See the Invisible Man," for example, focuses on how a futuristic form of punishment affects the individual and the surrounding society. Ursula K. LeGuin is a noted author of sociological science fiction. (For more information, see How to Write Soft SF, by Penny Ehrenkranz.)
Space opera, like Western "horse operas," often involves good guys "shooting it up" with bad guys (who may be aliens, robots, or other humans) in the depths of space or on a distant planet. Space operas (of which Star Wars is a classic example) don't worry about scientific implausibilities; technical explanations tend to be vague ("You see, Bob, if you fold space just so...").
Time travel was popularized by H.G. Wells with The Time Machine (1888), though Edward Page Mitchell wrote "The Clock that Went Backwards" seven years before that. Characters travel to the past or future, or are visited by travelers from either end of the spectrum. Topics range from "Let's go see what the Pleistocene looked like," to issues of paradox (what if you traveled to the past and killed your own grandfather?) and "tampering" (could stepping on a butterfly in the Paleolithic profoundly alter the entire future?). A variant of this subgenre is the "alternate universes" theme, in which each change in the timestream spins off a new universe.