Concentration camps were more along the lines of camps that were used to hold and secure people, usually civilians, that were unwanted or suspicious to the country holding them. Many people like to attune it to just Jewish people in Nazi Germany, but concentration camps have been used by the US against Native Americans and Japanese people, they were also used by the Japanese during world war II as well as the Russians.
A Death camp is exactly what it sounds like, usually used only to eliminate evidence, humans, or unneeded Prisoners of War, traitors, or civilians. Most death camps that were recorded in history came from Nazi Germany and Japan during WWII.
Mercenaries were groups of people that pretty much had no loyalty to a single nation. The fought along with nations that paid them (roman empire in this case).
