Name: Nineteen Eighty-Four, by George Orwell
Written in: 1949
Set in: ...1984.
Why it's a dystopia: The world is broken up into three super-dictatorships, two of which are always at war with the third; everybody is spying on everybody else, everybody lives in more or less abject poverty, and eventually the secret police comes, takes you away, tortures you until you break, and then shoots you. Also, you can't turn off your television set. Everything in that first sentence may or may not be true, by the way, even in the context of the book.
Why it's significant: You probably read this book in high school. Also, every politically-motivated online idiot on the Left will eventually reference this book while whining about whatever the Right's done, or thought to have done, or is incorrectly alleged to have done this week (don't smirk; there's a similar problem on the Right with regard to Atlas Shrugged). Nineteen Eighty-Four has also more or less interjected itself into our popular culture, and to a certain extent our language. All in all, it's probably the most mainstream piece of masochism porn in Western literature.