There was a border between east and west Berlin, but people could cross the border freely until 13th August 1961. The leaders of the Communist parties of the Commecon decided to close the open border between East and West after a 3 day meeting in Moscow.

There are four generations of Berlin were built from 1961 to 1975. The first generation of Berlin Wall was erected by East German government on August 13, 1961 during night time. Barbed wire entanglement and fences were installed through Berlin to close the border. Two days later (August 15, 1961), concrete elements and large square blocks were used to finish up the first generation of the Berlin Wall.In June 1962, to prevent citizens from escaping to the West, East German Government built a second generation Wall .

In 1965, the third generation of Wall were built to make it even harder to cross. The wall is consisted of concrete slabs between steel girder and concrete posts with a concrete sewage pipe on top of the Wall.

In 1975 the third generation of Wall was replaced by the fourth generation with new concrete segments.

For more about Berlin Wall, you can visit the following sites:
en.wikipedia.org/wiki/Berlin_Wall
www.berlinermauer.se/BerlinWall/bygg.htm
www.dailysoft.com/berlinwall/history/index.htm
library.thinkquest.org/20176/berlinwalltimeline.htm