One of my all-time fave writers is Ursala K Le Guin; I guess you could say she's somewhat of a hero of mine. In a great interview with Bill Moyer on PBS (video below), she talks about what she feels is the main difference between the genres of fantasy and the genres of science fiction.
According to Le Guin,
"Science Fiction doesn't contradict science, but it goes on and invents just as freely as fantasy does."
"Fantasy makes up its own rules; it's a much older genre; its roots are deeper." The happenings in a story of fantasy don't necessarily have to align with our current understandings of physics--laws of the world.
Isn't this such a wonderful way to look at these two genres? So often time they are lumped together as Science Fiction & Fantasy--with that misleading ampersand that, in my mind, seems to suggest some extremely close correlation.
Sure, they are similar genres, but each has its unique perspective and ways in which it critiques a universe.