The Treasure Room (also known as the Item Room) is a room without Monsters that contains an Item on a pedestal. All items in the Treasure Room draw from the Treasure Room Item Pool. Therefore, items from other rooms (Devil Room, Challenge Room, Angel Room, etc.) cannot be found, unless they are in multiple pools.
It requires a key to access. An exception is The Basement 1/The Cellar 1, which does not. The Basement/Cellar XL does require a key for both item rooms, however.