As the Nazis came to power, they began to construct special prison camps for housing people who disagreed with them. The inmates of these camps initially were mostly Communists and social democrats who were deemed enemies of the state by Nazi leaders. These people were almost never subjected to a trial, but were summarily thrown in the camps.
Initially, Jews were largely not held in concentration camps. Rather, with the passage of the Nuremberg Laws, Nazi authorities hoped that the mistreatment they suffered would cause them to leave the country. When this didn't happen, the policy of confining them to ghettos and concentration camps commenced. While a distinction should be made between concentration camps, most of which were in Germany, and the death camps that became instruments of the final solution, thousands of Jews, as well as other people deemed undesirable by the state, perished in the brutal conditions of these camps, and many more were summarily executed. This is how the concentration camps decreased the Jewish population.