I like a lot of the definitions given for Sci Fi so far. Of course, the idea of it being the genre of "what if?" seems pretty spot on and I really liked the idea that somebody else gave in class of it being about scientific development and how those developments affect the world in which they exist. Going deeper into that, I feel like Sci Fi is often an attempt to warn of the dangers of scientific development. This is not always the case, but it seems to be a common trend. It explores the ramifications of what all of the possible scientific advancements and "what ifs" could lead to. These worlds often seem to serve as metaphors for our own world. This idea of cautionary tales also lends itself to aspects of sci fi that do not pertain necessarily to scientific advancement. Stories of alien attacks often seem to be a metaphor for fears of the unknown, or even racial tension and wars between two different countries.
Personally, I actually have very little experience with Science Fiction, especially Science Fiction literature, hence the lack of a fun title. Most of the Sci Fi that I've been exposed to is filmed. This includes movies and television. I am an avid television watcher, and I'm a huge fan of all things Joss Whedon. A lot of Whedon's work, such as Buffy the Vampire Slayer and Angel, would probably be considered more Fantasy than Sci Fi, but both had clear Sci Fi elements. However, I would definitely consider some of Whedon's other works, such as Firefly, Dollhouse and Dr. Horrible's Sing-Along Blog to fall into the Science Fiction category. Other shows that I've watched that include Sci Fi elements include Lost, Heroes and Doctor Who.
Besides these examples, here are a couple of other things I immediately thought of when asked to name some examples of science fiction: Orwell's 1984, works by H. G. Wells, and even the Sookie Stackhouse novels by Charlaine Harris, or the show based on the books, True Blood. These books are another work that would probably be widely considered Fantasy, but the main background of the story is that vampires have just come out in the open because of a brand new scientific development of synthetic blood that makes it no longer necessary for vampires to feed on humans. That seems pretty Science Fiction-y to me.