Short answers, pulled from the story.
East German soldiers began erecting the Berlin Wall on the night of the 12th of August 1961. This concrete barrier sealed off East Berlin from the West and trapped millions of citizens within a socialist state.
The Socialist Unity Party of Germany formed on the 21st of April 1946 through a forced merger of the Communist Party of Germany and the Social Democratic Party of Germany. The SED ruled East Germany until 1990 with virtually total control over policy.
The Ministry of State Security operated a vast network of informants that numbered approximately 109,000 active agents by the end of the GDR. Between 300,000 and 500,000 victims suffered from direct physical torture and gross human rights violations under the Stasi.
A few sections of the Berlin Wall opened on the 9th of November 1989, allowing thousands of East Germans to cross into West Berlin and West Germany. This event marked the beginning of the end for the German Democratic Republic.
The five states of East Germany officially joined the Federal Republic of Germany on the 3rd of October 1990. This date marked the end of the German Democratic Republic and the unification of East and West Berlin as a third city-state.