A common piece of playground equipment. It basically consists of a ladder placed horizontally from 4-6 feet above the ground, supported by legs at each end. The legs may be made of ladders themselves to aide in climbing. As children are ment to hang from the crossbars, the rungs are usually made from narrow metal pipes.
The goal is usually to swing along the monkey bars from one rung to the next, making it all the way across without touching the ground. They are often combined with jungle gyms, tree houses, and other playground equipment.
In modern playgrounds they are being replaced by a type of 'zip cord'--a handle that you hang from and jump towards the other side. The handle is mounted on ball bearings, so you can cross the distance with minimal arm strength.