The numbered tattoos that have today become an identifying mark of Holocaust survivors originated in Auschwitz, the largest Nazi concentration camp in Europe. There, incoming prisoners went through the infamous selektion (selection process). An SS officer would sort the prisoners into two lines: those sent to the right were immediately killed in the gas chambers, those sent to the left were put to work in the forced labor camps. After their heads were shaved and their personal possessions removed, the prisoners were officially registered. Beginning in 1941, this registration consisted of a tattoo, which was placed on the left breast of the prisoner; later, the tattoo location was moved to the inner forearm. It was not only Jews who were marked: all prisoners other than ethnic Germans and police prisoners were tattooed. These tattoos were just one of the ways in which the Nazis dehumanized their prisoners. Despite the perception that all Holocaust prisoners were given tattoos, it was only the prisoners of Auschwitz after 1941 who were branded this way.
Sources: Gutman, Israel. Encyclopedia of the Holocaust. "Auschwitz." Volume 1. NY: Simon and Schuster. 1990.
Bard, Mitchell G. The Complete Idiot's Guide to World War II. NY: MacMillan, 1998.