A "fantasy" is more or less something you wish to see happen, or dream you would like to come true. It is not a reality. An "illusion" is like a trick your mind plays that you think you see something abstract or something that does not seem real or actually there. It is about perception in what you are looking at - and they are quite fun. check this site out to understand illusions: http://www.123opticalillusions.com/

A "fantasy" is something you imagine that is impossible or improbable. A fantasy involves wishing for or dreaming of something that will never happen. For instance: "her plans for their new house had moved into the realms of fantasy". An "illusion" is when you wrongly perceive or misinterpret a sensory experience. For instance: "the surface is striped to create the illusion of wood".