A Pullman bed is a bed that pulls down from the ceiling or folds out from the wall of a cruise ship cabin, allowing that cabin to sleep more people with less floor space.
Named for inventor George M. Pullman, Pullman beds were first introduced in the sleeper cars of trains in the late 1850s. Although the beds were considered luxurious at the time, their modern counterparts are known for being small and less comfortable than standard beds. Pullmans often have weight limits, and usually there's not much clearance between the top of the bed and the ceiling. That means the bunks are ideal for children or shorter, lighter adults who won't hit their heads when sitting up in bed.
Generally these fold-out, pull-down beds are ideal for families with children or those who are traveling on a budget and want to squeeze more (or all) of their travel party into one cabin.
As far as setup goes, upon request, your cabin steward will take care of lowering the beds (and corresponding access ladders) each evening and raising them again each day.
Pullman beds can be found on most mainstream ships, but the configurations vary by vessel, so check the deck plans or ask a travel professional before booking if you need an extra berth in your stateroom.