Basicaly a ladder placed horizontally from 4-6 feet above the ground, supported by two legs at each end. As children are ment to hang from the crossbars, the rungs are usualy made from narrow metal pipes.
The goal is usualy to swing from one rung to the next, making it all the way across without touching the ground.
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, and so you can cross the distance with minimal arm strength. | 
