They're fairly similar, a lot of times. For example, Star Wars is a fantasy movie that often gets lumped in with sci-fi. But for me, low fantasy, like Pratchett. Fuck that Wheel of Time type shit.
Science fiction is just the long way around to the same destination.
Fantasy: "Furries are real"
Science fiction: "The Federation used technology Captain Kirk stole from the Romulans to genetically engineer furries into existence"