The Auschwitz-Birkenau concentration camp was the largest German extermination camp during the Nazi era. It was built in 1941 three kilometers away from the main camp Auschwitz I and was located near the city of Oświęcim, renamed Auschwitz, in the district of Bielitz, which was annexed after the military occupation of Poland and established as an administrative unit.
The name "Auschwitz" became a symbol for the Holocaust. Of the more than 5.6 million victims of the Holocaust, around 1.1 million people, including one million Jews, were murdered in Birkenau. Around 900,000 of the deportees were murdered or shot in the gas chambers immediately after their arrival. Another 200,000 people were murdered by the SS through illness, malnutrition, mistreatment, medical experiments or subsequent gassing. Most of those murdered came from Belgium, Germany, France, Greece, Italy, Yugoslavia, Luxembourg, the Netherlands, Austria, Poland, Romania, the Soviet Union, Czechoslovakia and Hungary.
Today, many parts of two of the large concentration camps are still preserved or have been added true to the original. They are a publicly accessible part of the Auschwitz-Birkenau State Museum, Holocaust memorial and Jewish cemetery on the site of the two former concentration camps I and II. This museum is also a memorial, international encounter and Holocaust research center. It was declared part of the world cultural heritage by UNESCO under the name Auschwitz-Birkenau - German National Socialist Concentration and Extermination Camp (1940-1945).

Source: http://de.wikipedia.org/wiki/KZ_Auschwitz-Birkenau