Extermination camp
When people arrived at these camps, they would go through a process called "selection."  Nazi guards would choose which prisoners would be kept to work as slaves, and which would be sent right to the gas chambers to be killed.  Usually, the Nazis sent elderly people, sick people, and very young children right to the gas chambers.  Healthy people were often kept for slave labor, and were forced to work until they were too sick or starved to work as fast as the Nazis wanted.  Then the Nazis would "select" them for the gas chambers.