Internment Camp 371 was a Dominion mining and, later, a prison facility located on an asteroid orbiting a gas giant planet. In the camp, Jem'Hadar weapons were always set on maximum, which would vaporize prisoners for the smallest infractions.
The camp was originally built on the asteroid as an ultritium mine. It contained several barracks to house miners, each with a separate life support system. The whole habitable area was covered by an artificial biosphere, beyond which was lethal. There was at least six barracks, each able to hold about ten miners.
The Dominion later converted the camp into a prison by 2371, using the barracks as prison cells for multiple prisoners. The camp, led by the Vorta Deyos and Jem'Hadar led by Ikat'ika, held many survivors of the Battle of the Omarion Nebula following the destruction of the Romulan/Cardassian fleet, including Enabran Tain. It also held people who were replaced with Changeling infiltrators such as Julian Bashir and Martok. The Jem'Hadar would put prisoners into isolation if they misbehaved. They also fought against prisoners, especially Klingons, in order to learn their fighting styles. As noted to incoming prisoners, the camp did not have any watchtowers or fences - being located on an airless asteroid, there was simply no "perimeter" which prisoners could try to escape beyond.
While Tain was held, he spent over a year to modify one of the old life support systems in his barracks to create a subspace transmitter. The transmitter was used to send a message to Elim Garak. When Garak was captured attempting to track down the signal in 2373, he and Worf were sent to the camp, where they discovered Tain as well as Bashir and Martok. Tain died at the camp shortly after, and Garak was able to change Tain's modifications to allow the group to contact their runabout left in orbit, and escape. (DS9: "In Purgatory's Shadow", "By Inferno's Light")
Several months later, General Martok noted that his time in Internment Camp 371 had "dulled" his reflexes. Worf himself told Benjamin Sisko of his experience of tova'dok with Martok in the camp. (DS9: "Soldiers of the Empire")
In a Section 31 holodeck program, Luther Sloan questioned the ease with which Bashir and the group had escaped Internment Camp 371. In the program, Internment Camp 371 was supposedly where Bashir became a Dominion sleeper agent after being brainwashed by Weyoun. Sloan suggested Bashir had been allowed to escape to gather information the Dominion would later collect. (DS9: "Inquisition")
Background information Edit
Internment Camp 371 was conceptually based on a Nazi prison camp from the film The Great Escape. (Star Trek: Deep Space Nine Companion, p. 422) In the script of "In Purgatory's Shadow", the camp is described thus; "This is a domed over area on the asteroid surface. It serves as the central area/prison yard for a Dominion Internment Center [....] This facility is for an elite group of P.O.W.s, people who the Dominion is especially curious about." 
The exterior of Internment Camp 371 was designed by Illustrator John Eaves, who planned it in a concept sketch. (Star Trek: Deep Space Nine Companion, p. 423; Star Trek Encyclopedia (4th ed., vol. 1, p. 214)) Stated Visual Effects Supervisor David Stipes, "That was a model that John Eaves built, and we shot it over at Image G." (Cinefantastique, Vol. 29, Nos. 6/7, p. 40)
The look of the facility's interiors was added to by Production Designer Herman Zimmerman, Set Decorator Laura Richarz, Construction Coordinator Thomas J. Arp, Property Master Joe Longo, as well as other members of the creative staff. The main part of the Dominion prison was built as a large set, on Paramount Stage 18. The same set even included a crawlspace area containing the sabotaged life support system; this tightly confined place was adorned with fiber-optic lights and a mix of tubes, and was painted with drab colors. "It was very grim," Richarz said about the camp. "Everything was dark gray and kind of dirty." As usual, the group devised some creative uses for commonplace items, transforming CD racks into light fixtures for hanging above the beds in the prisoner barracks, and turning plastic Christmas tree stands into plant-ons to stick on the walls. (Star Trek: Deep Space Nine Companion, pp. 427-428)
In the game Star Trek: Deep Space Nine - The Fallen, the exterior and interiors of Arduria station resemble Internment Camp 371.