Science Fiction as a Genre

What is Science Fiction?   Explain some of the elements that distinguish this genre from others.  Certainly we cannot reply totally on The War of the Worlds for our definition of Science Fiction, although the literary account and the original film have illustrated some of the features we find in the genre.  Explain your experience with Science Fiction - what are some of the stories you've read - what are some of the films you have seen?  Does Science Fiction have any redeeming qualities as far as social or political significance or does Science Fiction simply entertain us?