BDSM cells tend to be much smaller that prison cells, as the captive will not usually spend more than a few hours in the cell at any one time, although in a few rare cases captives have spent very much longer in a cell as part of an extended scene or long term BDSM relationship. Cells create a 'space' of confinement and provide a barrier between the captive and the captor. The fact that the captive cannot see the captor can be unsettling for them and create a sense of yearning for their return. Cells are a very effective way of preparing a sub for a scene and creating a division between the real world and the fantasy world of the scene.
While a cell should not (in theory) require the captive to have any additional bondage to prevent them escaping, it is common for additional restraints to be used on the person to enhance the sense of helplessness and loss of control. The use of hoods, ear plugs and darkness can all be employed to effect sensory deprivation and to enhance further the sense of loss of control.
Many dungeons will have a cell as part of their facilities. It is also possible to create cells in ordinary houses by clearing out cupboards or using cellar space in older houses (Balzac uses an old coalhole under the front steps as a cell). Baby monitors or webcams can be used to monitor the captive without letting them know they are being observed.