When you here that term is usually refers WWII and the Nazi's. Concentration camps were basically huge "prisons" or barracks where the Nazi regime placed people deemed undesirable to the Aryan race. It was were a place where these people were labeled by numbers and all there possessions were taken away. There was very little food, and poor sanitary conditions. 11 million people were killed in these camps. 6 million Jews were the greatest number killed. The other 5 million consisted of other types of people that the Nazi's considered undesirable- such as Roma Gypsies, persons with disabilities, homosexuals.