Science Fiction is an adventure, mystery, drama (heck, any genere) that has some basis in a scientific fact or wild theory (faster that light space travel is a great example). The level of research, or reality towards the science determines what kind of science fiction it is. For example, Star Wars uses a few facts (or crazy theories if you prefer), such as the idea of FTL travel through "
hyperspace"; but since it's all pretty improbable, star wars type stuff is considered "
space opera". Just unplug your analytical mind and watch the pretty lightsabers.
If the science is sound and well-researched, like some of Ben
Bova's (one example, there are many other authors) writing, than it's considered "hard" science fiction and calls on a broad knowledge base of
popular science. Knowing a little bit about the physics or chemistry (or history sometimes) helps you appreciate and enjoy this kind of science fiction.