A pulley is a simple machine that does a job a single human being may not be able to do. A pulley is straightforward. All you need is a wheel and a piece of rope looped around the wheel. The wheel needs to be secured in place — preferably a high place if you're trying to lift a heavy object — if it will work properly. One end of the rope is fastened to the object you lift, and you pull on the other end. The rope glides over the wheel, which turns in the same direction. As the wheel turns, it helps reduce the amount of force needed to lift the object. An example of a pulley that you may have seen, or maybe even used, is a flag pole. In this instance, you can stand on the ground and move the flag far up into the air by pulling on the chain or rope.

Source: Elephango