I thought I would post the policy from the US Library of Congress regarding how they define Fantasy and Science Fiction. While many may feel they are not the final authority on this subject, I thought it would be interesting to have the opinion of the world’s largest library on this matter (actually, this policy might make things a little more obscure). Here it is:
A. Fantasy
Fantasy includes the sub-genres of science fiction, horror and adaptations of traditional myths. The distinguished writer, Arthur C. Clarke, has stated that "any sufficiently advanced technology is undistinguishable from magic." (Omni, April 1980, p. 87.). This view is borne out by the fact that the distinctions between science fiction and the various other sub-genres of fantasy are indeed blurred at times and usually artificial. In fact, many authors in the genre frequently cross these artificial barriers in mid-work or in mid-career. Publishers, furthermore, often confuse these sub-genre identifications even further by failing to differentiate among them.
Publishers do, however, frequently identify books in these various sub-genres with tags which usually appear on the spine or cover of the individual books stating that they are specifically fantasy, horror, science fiction, etc. These tags may be very useful in identifying materials whose precise classification is doubtful or subject to various interpretations.
Although difficult to define with precision, fantasy usually requires a willing suspension of disbelief. Works in its various sub-genres often 1) adapt, rework, or provide an alternate telling of a myth or folktale; 2) involve an alternate reality or alternate universe; 3) rely on a displacement of time or space; or 4) make use of elements of the horrific, supernatural, paranormal, or the occult.
B. Science fiction
In addition to sharing any or all of the general characteristics listed above for fantasy, science fiction usually 1) is speculative in nature; 2) assumes change as a given; 3) projects a story-line into the future or into an alternative reality or history; 4) explores a problem in technology, culture, philosophy, etc. beyond its current state; and 5) presents an atmosphere of scientific credibility regardless of the reality. Not all science fiction 1) takes place in the future; 2) involves space travel; 3) describes technology beyond current reality; or 4) deals with alien cultures. However, these elements are common in this sub-genre and uncommon outside it.
More can be read here:
Library of Congress