I never knew the difference between sci-fi and fantasy until I had to teach it to my sophomores. This led to lots of (fun) research, so to save you some time, I'm going to give it to you short and simple:
Fantasy is magic and mythical creatures; stuff that doesn't and couldn't exist in our world.
Sci-fi is advanced technology (i.e. genetic engineering, virtual reality), space exploration and settings (i.e. aliens), and time travel (thanks to advanced technology); stuff that could exist in the world as we know it, but doesn't because we aren't that advanced yet.