I really like Rod Serling's definition, "Fantasy is the impossible made probable. Science Fiction is the improbable made possible." Basically science fiction is a genre that deals with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities.
Copyright by Winston Science Fiction (Alex Shomburg)
As a means of understanding the world through speculation and storytelling, science fiction has a background in mythology and precursors of the genre can already be found in Lucian's True History which dates back to 200AD. Not surprisingly science fiction is often called a literature of ideas. Its imaginary elements are largely possible within scientifically postulated laws of nature though some elements in a story may also be strictly imaginative speculation. And it's usually the perfect balance between established facts and fictitious elements, a blending of the known and the unknown, that makes a great science fiction novel.
Science fiction elements are many-fold including eg a time setting in the future, in alternative timelines, or in a historical past that contradicts known facts of history, a setting in outer space or on other worlds, characters like aliens, mutants, androids, or humanoid robots, a futuristic technology, scientific principles that contradict known laws of nature, eg time travel or faster-than-light travel, different political or social systems (dystopia, anyone?) or a post-apocalyptic setting, and even paranormal abilities such as mind control, telepathy, or teleportation.
Science fiction has plenty of sub genres. Genres concerning the emphasis, accuracy, and type of science described are Hard Science Fiction (a particular emphasis on scientific detail), Soft Science Fiction (focus on human characters and their relations and feelings instead of the details of technology), and Social Science Fiction (concerned with sociological speculation about human society). Themes related to science, technology, space and the future, as well as characteristic plots or settings include Apocalyptic and Post-Apocalyptic, Steampunk, Time Travel, and Space Opera, to name but a few.
How about you? Science fiction yay or nay? Any preferred sub genres? Dystopia, maybe? Let me know.