Science fiction encompasses a huge range of different types of stories.  More than it probably should, really.

Hard science fiction is considered by many purists to be the only true scifi.  In this type of story, all the rules of science are followed, or extrapolated from what is already known.  Sometimes these stories will have a scientific element that doesn’t seem possible, but, in the end, real science drives the narrative.

Some try to draw a difference between scifi novels that feature “softer” sciences like the various social sciences, and the novels that focus on the one true science: physics.  Personally, I’d put both into the same “hard scifi” category, as they’re both based on known science.

Something you’ll come to realize after pondering the above is that the majority of people have never seen a real scifi movie or read a real scifi book.

Take, for example, the new Star Trek movie.  While the general public would classify it as a scifi movie, it has more in common with fantasy stories than science.  Off the top of my head about the only thing in it that is based on real science is the scenes where it goes to dead silence in space (and it isn’t even consistent in that regard).  Most of the rest of the technology is pure fantasy.  This holds true for almost every major scifi-categorized movie.  They’re really just action-fantasy movies.  Not that there’s anything wrong with that, but I think it does give the public a wrong idea of what much of science fiction is about.

One of my favorite hard scifi novels of all time, Rendevous with Rama, may become a movie.  With David Fincher and Morgan Freeman, it would have a good chance of being a credit to the book.  Unfortunately, there are rumors that this project may not happen.