What's The Difference Between Fantasy And Science-fiction?

Two different genres. Fantasy mainly deals with trolls, dwarfs, knights, castles, gods, witches, fairies, enchantments from different mythology and an era long forgotten. Science Fiction mainly deals with, modern high-tech gadgetry. Some writers like to mix the two genres, but it seldom works out. I try to read one or the other and stay away from the mixed-genres.

Science Fiction most often takes place in a future time period and includes scientific elements. Fantasy is often set in an imaginary world setting that seems to be in a past time period and includes magical elements.
However, there are many sub genres of each including, but not limited to: Urban fantasy, steam punk, hard science fiction, dark fantasy, etc.