Post-apocalyptic narratives consist of stories that are set in a world or a civilization after the occurrence of a global Disaster, such as an extremely destructive War (often as a result of nuclear weaponry), plague, environmental collapse, failure of technology, alien invasions or Zombie outbreak. While they are often related to Science Fiction, it is not always the case.
Dystopian, coming from the Greek words δυσ- and τόπος (which mean "bad" and "place" respectively), is a film genre that describes a society, usually in the future, which is under control of a very repressive regime. The films are often characterised by violence, War, poverty, and suffering.