In it's typical
configuration the
bunk bed is actually multiple
beds, one arranged over the other, used where the people to
floor space ratio is too high for more than one bed.
Military applications usually involve more than 2 beds, such as the five person cots-on-a-chain on the
USS Alabama, or the three person beds in a cabinet on WWII submarines.
In the suburbs however, the bunk bed is the source of many prepubescent hijinks, and can make a really cool fort.