But you should probably also know about the other SF, Speculative Fiction.
Speculative Fiction as a genre term is often attributed initially to Robert A. Heinlein. While he didn't initially include Fantasy in his definition, the term has grown to include many genres.
Speculative fiction is an umbrella term encompassing the more fantastical fiction genres, specifically science fiction, fantasy, horror, supernatural fiction, superhero fiction, utopian and dystopian fiction, apocalyptic and post-apocalyptic fiction, and alternate history.
Because SF and Sci-Fi both refer to Science Fiction (in lesser or greater degrees), some abbreviate Speculative Fiction as 'spec-fic.'
In my own reading, I started reading more serious Science Fiction authors like Heinlein and Isaac Asimov, however, it was the discovery of Roger Zelazny that changed my preferences forever. Zelazny sometimes defined his writing as 'speculative fantasy,' indicating his awareness that he often trod a different path from more rigorous Science Fiction authors.
Roger Zelazny is the most notable of several speculative fiction writers who mix fantasy with science fiction. Zelazny brought stylistic consciousness, somewhat better characterization, mythical archetypes, humor, and deeper explorations of psychology to his novels.
Rebecca Weybright has another definition for Speculative Fiction that I quite like.
All fiction is to some degree concerned with speculation–asking the question ‘What if.’
Speculative fiction, however, asks the ‘What if?’ question in both broader and more specific ways. The main branches of speculative fiction are science fiction, fantasy, and horror, and there are many fabulous little hybrids and sub-genres of those three.There is a nice list of the many 'fabulous little hybrids here. Some of my favorite hybrids are cyberpunk, magical realism, and space opera.
To be very simplistic, science fiction goes beyond the boundaries of the known, real world through (you guessed it) science; fantasy does so through magic or other paranormal means; and horror does so by expanding the role of fear.
Finally, Charles Tan explains why Speculative Fiction is so cool—it's harder to pin down and allows room for genre surprise as a reader.
The term speculative fiction holds a special place in my heart because it is able to accommodate all the other genres under its umbrella. What I mean by that is that fiction written under it can include elements of romance, mystery, horror, etc. In a certain way, the same can be said about the other genres....And for added value, you can mix and mashup some of these sub-genres to make for a truly fantastic reading experience. But we can talk more about that next time. ;)
There's also another advantage to such a vague term as speculative fiction. The only expectation one comes into reading it with is that anything can happen. Take horror, for example. When you read a short story that's classified as horror, you're already preparing yourself for the horror element to pop up. Compare this to reading a story simply labeled as speculative fiction, where you don't know whether it'll tickle your sense of wonder or frighten you (or both). While there's something to be said about the technical craft of writing a horror short story, sometimes it's better to not know that it's a horror piece in the first place for it to have a greater impact.