An internment camp was a type of prison.
During World War II on Earth, the United States imprisoned its Japanese-American population in internment camps such as Manzanar. Based on this historical experience, Captain Jonathan Archer decided to liberate a Tandaran internment camp called Detention Complex 26.
The Tandarans established such camps during their conflict with the Suliban Cabal in the 2150s and interned Suliban there, regardless whether those were aligned with the Cabal or not. The camps were defended by phase cannons and Tandaran patrol ships, but the camp itself was not protected from unauthorized transporter activity. (ENT: "Detained")