After WWII, Germany was divided between the Allied forces into what became West Germany and the German Democratic Republic (aka GDR or East Germany). The dividing line went straight through Berlin. When it became clear that the GDR was not going to enjoy the same freedoms as that of their West German counterparts, people began migrating (or defecting) in mass numbers. To counteract the mass migration, the East German government erected a giant concrete wall along the political border in Berlin. Completed in 1961, the Berlin Wall separated families, ideas, the city, and the country.
But the wall came crashing down in 1989, reuniting Berlin and the country.
Today, there are still some sections of the wall standing in various parts of the city. The biggest section (and the most popular with tourists) lies on the border between Kreuzberg and Friedrichshain along the River Spree. Currently, there’s an open air gallery on one side and a photographic exhibition wallonwall.org on the other. It’s definitely worth a visit.