I was going to make a reply in my own words, but in searching for complementary data I found another person writing a better and more concise answer, which I will copy here instead:
"Science Fiction and Fantasy are both considered sub-genres of Speculative Fiction.
The essential difference is that science fiction is supposed to be plausible at the time - it requires little or no suspension of disbelief. It is generally (but not always) set in the future, and involves advanced technology which allows certain breaks from current limits.
Fantasy is traditionally set in the past, or in an alternate universe which resembles certain times in our past. It generally requires more suspension of disbelief - magic and the supernatural play a much larger part.
These aren't hard and fast rules, of course."