A dystopia takes place in a disastrous universe in the future, likely imagined yet one that hit home. They tend to critic a political aspect already planted that will develop itself in the future to create inequalities or unfairness. Generally, propaganda and unfair surveillance serve as tools to maintain order inside those systems.
The main protagonist find himself trapped emotionally and physcilaly in this society. We usually learn about the changes that occurred thanks to him and see the madness unfold through their eyes.
Some well-known dystopias: 1984 (George Orwell), A Clockwork Orange ( Anthony Burgess) All the Birds in the Sky
Note: some YA book series do counts as dystopias such as Hunger Games.