Sci-fi differs from fantasy by the inclusion of real (or real-feeling) science. Some books are set distant future (think time travel and space exploration) while others focus more on sociology and anthropology and predict the future of the human race. Are there ETs? It's sci-fi. Is there an oppressive government or was the world destroyed by climate change or war? Sci-fi. Has technology run amok? Yep, it's sci-fi!