Sci-Fi stands for Science Fiction. It's novels, stories, movies ASO with a heavy content of future/foreign science/technology discoveries. Space travel is just about required. Star Wars and Star Trek...
Yes and no, it is often used in the subordinate clause sense- by itself science fiction, but the usually grammatically adrot Funk and Wagnalls encylopedia will say- Brdbury writes powerful...
Sci-fi nerds--more commonly, they refer to themselves as sci-fi geeks--are a group of people who obsessively follow one or more fandom(s), such as Star Wars, Star Trek, Firefly, Battlestar Galactica,...