Why does it exist?
Historically, concentration camps are intended to "re-educate" through the work of people who are refractory to the totalitarian ideology of the regime (political opponents, resistance fighters, etc.). In fact, these are places of repression providing a free workforce.
And today, what about it?
Several researchers and NGOs consider, for example, that the conditions of internment of hundreds of thousands of Uyghurs in Xinjiang (China) present a concentrationary character.
Simply a prison. In a concentration camp, political opponents are usually locked up, who have not been given a fair trial. Detainees do not benefit from the common rights of a litigant (principle of contradictory, presumption of innocence, etc.).
An internment camp. In an internment camp are gathered people defined as 'undesirable', placed away from the rest of society and put under surveillance.