An internment camp was a type of prison.
On Earth during World War 2 the United States imprisoned its Japanese-American population. Having learned from history,captain Jonathan Archer after learning of similar camps operated by the Tandarans decided to make an exception in his policy not to interfere with other cultures and liberated one.
These Tandaran camps, where they held Suliban, even innocent ones, during their conflict with the Suliban Cabal in the 2150s, were defended with phase cannons and Tandaran patrol ships. The camp itself was not protected from unauthorized transporter activity. (ENT: "Detained")
The Dominion operated internment camps to hold enemies of the Dominion. In the run up to the Dominion War, One such camp was Internment Camp 371, where the law was that "there is no release, no escape... except death." (DS9: "In Purgatory's Shadow")