Science fiction - fiction usually set in the future or in an alternate history, dealing with the effect of the future, technology, or some combination of both. Often abbreviated as "sci-fi" or called "speculative fiction."
Dystopia - fiction set in a world that is designed to be oppressive/totalitarian/unlivable as possible.
Utopia - fiction set in a world that is designed to be as perfect as possible. Infrequently written today.
Post-holocaust - fiction set in a world that has experienced a disaster or catastrophe. Examples include the aftermath of nuclear war, a plague, volcanic eruption, ozone depletion, or anything else that causes calamity. A post-holocaust story is not necessarily a dystopian one, although the two may overlap.
Post-holocaust fiction is commonly labeled as dystopian today, but post-holocaust scenerios often create a world that is grim on the face. People attempting to live and make the best of such a scenerio isn't a dystopia, but people who are using the disaster as an excuse to seize power are.
As another note, I'd rather cut off my own ears than reduce a book to a rating. I write the review and tell you what's good and what isn't.