Most people are familiar with the names of the major concentration camps - Auschwitz, Buchenwald, Dachau, and Treblinka, for example - but few realize that these were not the only places where Jews and other prisoners were held by the Nazis. Each of the 23 main camps had subcamps, nearly 900 of them in total. These included camps with euphemistic names, such as “care facilities for foreign children,” where pregnant prisoners were sent for forced abortions.
When the U.S. Holocaust Memorial Museum first began to document all of the camps, the belief was that the list would total approximately 7,000. However, researchers found that the Nazis had actually established about 20,000 camps between 1933 and 1945.
These 20,000 camps were used for a range of purposes including: forced-labor camps, transit camps which served as temporary way stations, and extermination camps, built primarily or exclusively for mass murder. From its rise to power in 1933, the Nazi regime built a series of detention facilities to imprison and eliminate so-called "enemies of the state." Most prisoners in the early concentration camps were German Communists, Socialists, Social Democrats, Roma (Gypsies), Jehovah's Witnesses, homosexuals, and persons accused of "asocial" or socially deviant behavior. These facilities were called “concentration camps” because those imprisoned there were physically “concentrated” in one location.
Millions of people were imprisoned, abused and systematically murdered in the various types of Nazi camps. Under SS management, the Germans and their collaborators murdered more than three million Jews in the killing centers alone. Only a small fraction of those imprisoned in Nazi camps survived.
Sources: U.S. Holocaust Memorial Museum