A Dystopia is the opposite of a Utopia, and can mean a Utopia that has become corrupt. This can be seen in George Orwell's book 1984. A dystopia often has many problems, such as poverty, pollution, or a cruel ruler.
A dystopia is different from an anti-utopia. An anti-utopia is a place that is supposed to be a beautiful and perfect utopia, but where something has happened to ruin it.