From New Dictionary of the History of Ideas:
Dystopia is utopia's polarized mirror image. While utilizing many of the same concepts as utopia—for example, social stability created by authoritarian regimentation—dystopia reads these ideas pessimistically. Dystopia angrily challenges utopia's fundamental assumption of human perfectibility, arguing that humanity's inherent flaws negate the possibility of constructing perfect societies, except for those that are perfectly hellish. Dystopias are solely fictional, presenting grim, oppressive societies—with the moralistic goal of preventing the horrors they illustrate.