A Concentration camp is basically a place where a large number of people are confined or imprisoned. This imprisonment is not the result of any kind of trial, in fact it is not even under any law. That is why prisoners in such concentration camps live a very dreadful life with long hours of labor and torture and without any basic necessities.
Such camps are usually established for political reasons. Normally they are set up during times of war and the population of the conquered nation are kept in such camps. However, the prisoners are selected based on a certain criteria such as their religion, ethnicity, any kind of illness, political opponents etc. There is no legal evidence of any kind of crime against them. In fact most of these people are completely innocent.
The history of the world is full of such camps that were set up by different nations. It includes the most famous Nazi concentration camps that held Jews. Then the US concentration camps for Cherokee, Native Americans and most currently Talibans. And the Soviet concentration camps during World War 2.