What is science fiction? From a "literature of change" to a device to explore "cognitive estrangement" or a "mythos of scientism," (or, conversely, to attack scientism), Machina Memorialis supplies several thoughtful definitions from prominent writers in the genre and links to many, many more.
My favorite quote in the larger list is attributed to Damon Knight:
What we get from science fiction---what keeps us reading it, in spite of our doubts and occasional disgust---is not different from the thing that makes mainstream stories rewarding, but only expressed differently. We live on a minute island of known things. Our undiminished wonder at the mystery which surrounds us is what makes us human. In science fiction we can approach that mystery, not in small, everyday symbols, but in bigger ones of space and time.