Pandorica

The Pandorica was a prison box, hid under Stonehenge, constructed to hold the most feared organism in the history of the Universe. It was mentioned by Prisoner Zero to the Doctor and told him that it would open and that silence would fall. (DW: The Eleventh Hour) River Song also mentioned the Pandorica to the Doctor, telling him she would next see him there and that she remembered it well. The Doctor believed at that time that the Pandorica was a fairytale. (DW: Flesh and Stone) 

The Pandorica turned out to be a prison, built possibly by the Alliance for the Doctor to stop him from destroying all universes (the most feared organism in all the universe). Their conclusions held that the Doctor would be responsible for the destruction of existence itself and thus used the lure of the Pandorica in order to entrap him. There were many layers of security in the Pandorica including deadlocks, time stops,metal lions and matter lines. (DW: The Pandorica Opens)

Exterior
The Pandorica looked much like an image of Pandora's Box found in the bedroom of Amy Pond. Since they used her memories to lure the Doctor there, the Alliance possibly designed it to look like this.

Interior
The interior was a small room with a chair in it, intending to be a prison. When someone was placed on the chair, the exterior would close, sealing them in, and many forms of security would stop the prisoner escaping.

Known Prisoners
The Doctor was put in the chair by the Alliance to stop him from destroying every universe. (DW: The Pandorica Opens)