Completely surrounding West Berlin, the wall effectively sealed off a section of Germany’s largest and most important city from the rest of East Germany, making it an island trapped in a country that did not govern it. Home to 2 million people, West Berlin was affiliated with—geographically, historically, ethnically and linguistically—but not legally a part of the Federal Republic of Germany (West Germany). As a result of post-war partitioning, it was essentially a city without a country.
Collectively, the Berlin Wall and the larger border between east and west were called The Iron Curtain (in the west, at least), because they symbolized the fiercely guarded border between the democratic Western Europe and the Eastern Bloc nations associated with the USSR.
Immediately after World War II, millions of people emigrated from Soviet-occupied Eastern European countries to the West. By 1950, the Soviets imposed restrictions, trying reduce the number of young educated professionals leaving for opportunities in West Germany, France and beyond. Nearly 20% of East Germany’s population took part in that migration, many from the eastern to the western sectors of the partitioned city of Berlin. To stem the human tide, they put up fences and by 1961, the fences had been turned into concrete walls.
Once the Berlin Wall and the fortified borders went up, emigration stopped and didn’t start again for almost 30 years.
Here is how Wikipedia describes the Berlin Wall:
The Wall included guard towers lining large concrete walls circumscribing a wide area containing anti-vehicle trenches, “fakir beds” and other defenses.
In June 1962, a second, parallel fence some 100 meters (110 yards) farther into East German territory was built. The houses contained between the fences were razed and the inhabitants relocated, establishing an area called No Man’s Land, later known as The Death Strip. The Death Strip was covered with raked gravel, easily disturbed by footprints. Guards could spot someone running the wide, open space, and had a clear shot at anyone they saw.