Clearly, Science Fantasy contains elements of science fiction such as technologically advanced machines, space travel and planets and universes different to our own. However, Science Fantasy also includes the fantasy element where mysterious powers co-exist with technology which can do apparently ‘magical’ things.
Perhaps one way of making the distinction between Science Fantasy and Science Fiction might be to think of Star Trek as science fiction and Star Wars or Robert A Heinlein’ six-book Dune series as Science Fantasy. The defining factor, is around whether the technology and the science in general can be explained as a development of technology we current believe to be possible, or not. The point here is that when elements of the story are attributed to progressive developments of existing technology then this is Science Fiction. Conversely, when the story includes elements that cannot be attributed to known possible developments and improvements in technology then this is science fantasy. Alternatively, science fiction makes the implausible possible, while science fantasy makes the impossible plausible. Inevitably, this means that large sections of both genres can fall into either category.
Despite this fairly clear definition of Science Fantasy, above, this is a particularly controversial area of debate for Science Fiction and Science Fantasy fans. One of the main problems is the issues of authors intent. By this I mean, for example, Lord of the Rings has many elements of Fantasy - but were Middle Earth to be on another planet then it might be considered Science Fantasy or Science Fiction. Similarly, warp drive powered space travel is Science Fiction, but psychic powered travel is Science Fantasy. This is a debate that will run and run and definitions do matter. However, in as much as technological advances are hard to predict with any reliability, perhaps most Science Fiction should be considered Science Fantasy - at least until proven otherwise!