Concentration camps were mostly occupied by men who were criminals, homosexuals, political dissidents or other "undesireables" during the early days of the Nazi regime. They were not killing sites, and many won their release after serving time there.
The Holocaust is most famous for the extermination centers or death camps. Here, entire families were sent. Men and women who were able to work were selected for this purpose, and those who could not were killed the same day. Once the "workers" became unproductive they too were killed or died from overwork, disease or starvation.
While all were considered concentration camps in a way, not all of them were death camps.
