Modern Fantasy is a type of fiction that "breaks one or more of the rules that govern 'real' life as we ordinarily define it" (Cullinan).
Science Fiction, on the other hand, depicts "plausible events based on the reality of the universe as we know it. These events might not happen in our time or in our world, but they are logical extrapolations of known facts" (Cullinan).
References
Cullinan, Bernice E. and Diane G. Person. The Continuum Encyclopedia of Children's Literature. Continuum International Publ, 2005.