SF you say? Not fantasy? Yes. Avatar is the perfect example of Science Fiction as defined by ARRT in our Genre Fiction List (also available to NoveList subscribers as "Popular Fiction Checklist" on the right hand side of the start page)
Science Fiction is speculative fiction based on plausible extrapolation from our current understanding of science and the physical world. The appeal of the genre is often the intellectual exploration of traditional ideas in nontraditional settings. The best Science Fiction evokes a 'sense of wonder' in new worlds and new adventures. The genre defies precise classification because science fiction authors experiment with themes, styles, and frames, blending technology with sociological ideas or adventure.
If you have seen Avatar you know it fits this definition perfectly. Fantasy is based on magic to provide the speculative part. In Avatar, science explains everything. Cameron built a world, Pandora, in which all of the creatures and plants could have easily evolved to be the way they are. This is science we don't know about yet, but it is not based on magic in any shape or form.
After we left the movie, my husband talked about what a great book Avatar would make. He was thinking more of a nonfiction book explaining the flora and fauna of Pandora, like a DK guide, but I was thinking what a great series of novels this meticulously created world could spawn.
Obviously James Cameron agrees with me, as he plans to write a traditional novel telling the pre-Avatar story of Pandora. This could be the next big SF franchise to fill up your shelves. If nothing else, it will bring new patrons into the library.
Keep an eye out for it. And, if you want to understand the appeal of SF go see Avatar or any of the original Star Wars movies.
On the other hand, for a better understand of Fantasy's appeal you can turn to the Harry Potter or Lord of the Rings movies. After seeing one of each, fantasy and sf, you will be better able to distinguish them in the future.