If Fantasy games make up the majority of RPGs, Sci-fi and all it's variables certainly take the next largest chunk of content. Everything from Star Wars to Shadowrun to specific era sub-genre like Pulp to a plethora of hard science-oriented games can be in this category. Mostly Sci-fi games refer to science and technology-based settings or worlds where their science and technology is beyond our currently available levels of common use. Sci-fi can have 'magic' in the form of psyionics (mental powers), but it's not a must have and some people even abhor such things. Typical Sci-fi elements include high or strange science and/or technology such as laser guns or cybernetics, space ships, space travel, and possibly some strange, unexplainable things that are just beyond our understanding such as black holes or worm holes in space. Aliens, robots, and mutants are all certainly fair game as well, but typically good old fashioned humans are the central focus.