An example of a twin bed is a dorm bed in college that has room for only one person.
Some people, however, like to buy comforters bigger than their beds, especially if they have a twin bed.
Additionally, many cribs today are designed to grow with your child, and can convert from an infant crib, to a toddler bed, and finally to a twin bed for your older child.
A twin over full bed configuration makes it possible to have a full bed and an additional twin bed all in the same room.
Even the combination of a traditional bunk bed plus a single twin bed will take up more space than a triple bed configuration.
One great example is the Willow Twin Sleeper which opens into a twin bed.