It can be estimated that under Adolf Hitler, the Nazis targeted Jews to such an extent that 6 million Jews were killed, an estimate that translates to two out of every three Jews (Holocaust Museum Houston, "Education: FAQs Page").
However, Jews were not the only social group targeted by the Nazis. Supported by the theory of Social Darwinism, Nazis believed anyone of the Aryan race to be superior to all other peoples; hence, the Nazis targeted anyone they felt was "different" or weak. Their victims included "Jehovah's Witnesses, homosexuals, Social Democrats, Communists, partisans, Polish intelligentsia and other 'undesirables'" (Holocaust Museum Houston).
In some countries, Nazis first rounded Jews up into "400 ghettos" they had designated. The term ghetto stems from a medieval concept of placing restrictions on where Jews could live; during the medieval ages, ghettos were established all over Italy and both central and western Europe. During Hitler's regime, the Nazis rekindled the concept of establishing ghettos, especially in Poland. In Lodz, Poland, about 155,000 Jews were forced to relocate to the Lodz ghetto ("Ghettos under the Nazis").
Jews were also rounded up and taken to death camps, also primarily located in Poland. The Nazis built six death camps in Poland: "Auschwitz-Birkenau, Belzec, Chelmno, Majdanek, Sobibor, and Treblinka" (Holocaust Museum Houston).
Nazis also gathered for the Wannsee Conference in a villa in Berlin on January 20, 1942, three years after the start of World War II in Europe, where they discussed the "Final Solution of the Jewish Question" (Holocaust Encyclopedia, "Wannsee Conference and the 'Final Solution'"). The Final Solution, called Endlosung in German, called for the "systematic, deliberate, physical annihilation of the European Jews" (Holocaust Museum Houston; Holocaust Encyclopedia).