Let's do some free association. What comes to mind when you think about science fiction? Aliens? Spaceships? Robots? All the stuff Steve Jobs invented? Well, those things sure are a part of the sci-fi genre, but, as Ursula Le Guin said in her introduction to The Left Hand of Darkness, "science fiction isn't about the future […] Science fiction is metaphor" (Source, 27). Deep.
It's not about the aliens, it's about what the aliens mean.
You're probably most familiar with H.G. Wells and Ray Bradbury, two guys who laid the groundwork for what was to come. But Robert Heinlein, Isaac Asimov, and Arthur C. Clarke are considered the Big Three of science fiction writers—these are the guys who brought science fiction into its golden age. Their stories proved that science fiction was much more than UFOs, ray guns, and bikini-clad damsels-in-distress. In fact, it could be—gasp!—serious literature.
Once that was settled, other authors jumped on the scene. Science fiction had a tendency to read a lot like, well, science with a dab of fictional flavor. But William Gibson threw a wrench in things by writing a hard-boiled detective novel (Neuromancer) genetically infused with a dystopian future. His characters were anti-heroes at best, lowlifes in general, and they knew how to navigate society's underbelly. A vast difference from your run-of-the-mill Captain Kirks and Luke Skywalkers, that's for sure.
Chances are you know someone who could wax poetic about science fiction literature for days on end—though they might be otherwise occupied at some sort of conference at the moment. That's because it goes back even to the oldest work of literature ever, The Epic of Gilgamesh. But that's a story for another day.