The genre, to me, is a hybrid of fiction and nonfiction. It is nonfiction because it is based, generally speaking, on a form of science, be it currently existing or an idea that is based on a well founded theory. The science of it all appeals to the more literal thinkers and those who need the books they are reading to have some base in reality.
In terms of fiction, it is very obvious: the stories being told are a bit far fetched and highly unlikely to occur. Often, these books will be based on a world that does not exist, meaning that the author is world building. Also, the book is based on a science that does not currently exist or is in its very early days of development, allowing the author to really become creative and make something fantastic to give the reader something to imagine and form within their minds.
Science Fiction is a great genre that should be included in more english and culture classes because it is all about technology that may not affect us now, but has the potential to in the future, be it near or far.