Ray Bradbury's definition captures one important aspect of SF, but fails to encompass the whole. There is a common tendency to think that SF must be filled with spaceships, exotic aliens, and far away worlds. The fact of the matter is that some of the best SF takes place right here on Earth, in the past. One of the key elements to the genre is the exploration of humanity, be it past, present, or future. These are the questions that SF strives to answer.
Science fiction is the very literature of change.
In fact, it is the only such literature we have. SF great Frederik Pohl gives us this definition that again addresses the subject of change. As you can see by now, change is the catalyst that makes SF go. It is in this way that we can make less-biased judgments of our own world. Finally, Dick Riley seconds the statement I just made about looking in from the outside.