Fantasy- Normally generates around supernatural ideas revolving around magic (mostly, if not all the time). Fantasy is based purely on imaginative thoughts that have to do with myths and superstitions, and scenarios that will not and can not occur.
Science Fiction- Due to the 'fiction' part, it means that it hasn't happened, and that it's only a thought of what could happen. Science fiction is based on scenarios that may POSSIBLY occur. This means that it's not exactly "out there", and not too far-fetched (logically- people have different opinions). Science plays a big part in science (hence, SCIENCE fiction).
EDIT: People get science fiction and fantasy mixed up a lot because it's a hard distinction between reality, and fiction. Seeing as how neither seem likely in the eyes of a regular person (again, depending on who you are), it's difficult to seperate the two, because they seem so alike on a logical level.