A police state is a political condition where the government maintains strict control over society, particularly through suspension of civil rights and often with the use of a force of secret police. This implies that the control by the government contradicts the will of the people being controlled. Thus, a police state is inherently anti-democratic. It is similar to martial law.
The definitive literary treatment of a police state is George Orwell's novel Nineteen Eighty-Four, which describes a totalitarian régime that uses the excuse of constant war to permit police and security cameras to surveil the entire population.
Until the fall of the Berlin Wall, a classic modern police state was East Germany, or the German Democratic Republic (GDR). The country's secret police force, the Stasi (or Ministerium für Staatssicherheit) maintained an incredibly close watch over East German citizens, to the point where virtually every residential building, place of employment or place of leisure was home to at least one Stasi informant.