What was built in Berlin in 1961 to divide East from West Germany?

On August 13, 1961, the Communist government of the German Democratic Republic (GDR, or East Germany) began to build a barbed wire and concrete “Antifascistischer Schutzwall,” or “antifascist bulwark,” between East and West Berlin. The official purpose of this Berlin Wall was to keep so-called Western “fascists” from entering East Germany and undermining the socialist state, but it primarily served the objective of stemming mass defections from East to West. The Berlin Wall stood until November 9, 1989, when the head of the East German Communist Party announced that citizens of the GDR could cross the border whenever they pleased.

Source: History