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.
Star Wars, Star Trek, Battlefield Earth (the book) = "light" science fiction, space opera
Robert Heinlein's future history (Methuselah's Children), Ben Bova's Jupiter, or Asteroid Wars, Frank Herbert's Dune (a little fantastical, but the ecological background is neat)
= "hard" sci-fi