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.
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
An elaborate, suspenseful, and sometimes well-written fairy tale that typically focuses on paranormal events, aliens, technology, futuristic visions, space exploration, multiple dimensions, genetics, and other themes which require a good use of logic and scientific theories/facts.