The first Concentration Camps began in Germany in 1933. The very first one that they opened was called Dachau. Jewish people were arrested for no reason, had no human rights, and were sent to live under very harsh conditions. By the beginning of WWII, the Nazis already set up six Concentration Camps. In the camps, the prisnors were forced to work as free, back-breaking labor to help the Germans continue fighting the war including working in stone Quarries, coal mines and construction.
In all, there were 1,500 camps, mostly small and temporary but mainly there were 20 major camps.
In the end it is believed that as many as 6,000,000 Jews were killed.
How were the Jews treated? The Jews were treated horribly, in the Concentration Camps they would be forced in labor, the Jews would do back-breaking work for the Nazis to help them gear up to continue WWII. When the Jews simply could not work any longer they were killed. The Jews were also used to build construction work.
Adolf Hitler was the leader of the whole Holocaust, he was the one who ordered the Nazis to capture the Jews and send them to concentration camps where they made the Jews suffer, which often led to their death. source: scrapetv.com
Jewish people in a concentration camp. source:http://pmayorga.blogspot.com/2010/04/earthlings - 2.htm