Auschwitz II (Birkenau) was a combined forced labor and extermination camp. Trainloads of victims, predominantly jewish, arrived at the rail siding where the selection officer would chose life or death for the arrivals with a flick of his thumb (see the  War and Remembrance Video Clip for a dramatization). Those deemed unable to work, usually about 75% of them, were picked for death and sent directly to the gas chambers. This included most women and children as well as the elderly. The others, deemed fit for work, were sent to the overcrowded barracks (housing some 200,000 in a facility designed for 50,000) for duty as slave laborers while they were still able. When no longer productive, they also were consigned to the gas chambers. The gas chambers were disguised as shower facilities, where victims were forced to strip naked, often provided with towel and soap to fool them, and herded inside. When the chamber was sealed, the SS would drop Zyklon B pellets inside. This Cyanide based pesticide would take about 20 minutes to kill everyone; their screams could be heard outside the thick walls. SS guards would then make other prisoners don gas masks and retrieve victims' eyeglasses, hair, gold teeth, prosthetics, and jewelry; and haul the corpses to the cremation ovens. Eventually, there were not enough ovens to handle the workload and corpses were buried in large trenches and bulldozed over. The SS destroyed most camp records, making it difficult to determine the total number of victims. But according to German shipping records about 1.5 million were "evacuated" for killing. Perhaps double that number actually perished; about 90% of them were jews. The others included Polish intelligentsia and dissidents, Romani, Sinti, Jehovah's Witnesses, homosexuals, Soviet prisoners of war, or anyone else the Nazis considered as enemies of the Reich. The bulk of the deaths were in the gas chambers, but many also died from the brutal conditions - starvation, overwork, disease, summary executions, and medical experiments. For further information:  Link.