To me they are very different. SciFi is just that, fiction based on science, usually set in the future. Fantasy is anything BUT scientific, it involves magic and the supernatural and is often set in the past. As a scifi fan, it always bothers me that the shelves in the library that are labelled SciFi are full of stories about wizards and mystics. And judging from some of the cover art, they seem to be aimed at women, not geeky boys.