A pulley is defined as being one of the three simple machines: the pulley, the lever, and the inclined plane. The purpose of a machine is to help accomplish the term physicists describe as work. While the rest of us may casually refer to work as any exertion of effort toward a goal, physicists have a very specific idea of work. It is defined as the force applied to an object multiplied by the distance that object is moved. Pulleys fall into one of three categories.
A fixed pulley describes a pulley that is secured to a single spot. While the pulley's wheel will turn with the rope or chord that passes through it, the pulley itself will remain stationary. Because of this, the force exerted on the object on the opposite side of the pulley will be exactly the amount of force applied on the user's side of the pulley. This is described as having a mechanical advantage of one, because the amount of force you apply is precisely the amount of force the machine will exert on the object you are trying to move.
A movable pulley is a pulley that moves with the load you are moving, as opposed to a fixed pulley which does not move. However, unlike the fixed pulley which exerts only as much force on the object being moved as is applied to the machine, a movable pulley will multiple the force which the user applies to the machine in doing work on an object. This means that less force must be applied by the user to do the same amount of work, effectively making the work what might be described as "easier."
For example, if a man was using a fixed pulley to lift a box then he must apply a force which overcomes the force of gravity on the box. Since the fixed pulley does not multiply force, this means he must apply that amount of force himself. In contrast, if he was using a movable pulley, which does multiply force, then he would only have to apply a fraction of the force necessary to overcome gravity, and the machine would multiply that into the sufficient amount.
The compound pulley system is a combination of fixed and movable pulleys. While this system has the greatest multiplication of user force, the system itself takes up space and involves moving the object a longer distance.