DDR existed from 1949 until 1990. Politically it was a dictatorship disguised as a democracy governed by the SED who took their orders from Moscow and was responsible for several thousands of deaths.
Stasi was SED's secret police. It was founded in the 1950's and occupied at least 100.000 people on full time and approximately twice as many informants. The aim was to spy on the citizens of DDR and silence any kind of criticism against the government.
Hohenschönhausen was from 1951 until 1989 DDR's well known prison where political prisoners were kept and interrogated as enemies of the state. Now, it's a museum with guided tours.