Ian The Poet
Wizard Time Traveller
Science fiction is fiction based on science, often involves future technology and strange worlds. It often prides itself on being scientifically feasible.Fantasy covers a wide spectrum of possibilities: paranormal (ghosts, demons, etc.), sword and scorcery, witches and wizards, a lot of quest novels with sword type warriors, often with supernatural/magic elements or overtones. And much, much more.
So, simply and generally put: Science fiction starts out based on science ideas. Fantasy starts out based on a world that includes magic or the supernatural.
And, oh yes, the two can be combined. There's a tremendous amount of variety.
I generally see them as two different genres, but they do get blended quite often, Fantasy has elements of the supernatural, it generally has magic, and other fantastic elements, talking animals, fantastic locations and different species, such as elves, dwarfs and trolls. Prime examples would be Harry Potter, The Lord of the Rings, and the Chronicles of Narnia.
Science Fiction is based more on Technology, it may have fantastic elements, but they are rationalized by technology, It may have space travel, other planets, aliens, and technologies beyond our own. Some examples would be Star Trek, Alien, and Transformers.
I would consider Star Wars, with it's use of the magical "Force" to be a blend of the two, even though Lucas added a technological explanation in the later movies.
The two genres also share many of the same fans, which is why they are often lumped together.
Science Fiction - its imaginary elements are largely possible within scientifically-established or scientifically-postulated laws of nature (though some elements in a story might still be pure imaginative speculation). Exploring the consequences of such differences is the traditional purpose of science fiction, making it a "literature of ideas".[1] Science fiction is largely based on writing entertainingly and rationally about alternate possibilities[2] in settings that are contrary to known reality.