A movable  pulley  is a pulley that moves with the load.

The movable pulley allows the effort to be less than the weight of the load. The movable pulley also acts as a second class lever. The load is between the fulcrum and the effort.

The main advantage of a movable pulley is that you use less effort to pull the load.

The main disadvantage of a movable pulley is that you have to pull or push the pulley up or down.

If you add a second  pulley, the amount of effort to lift the heavy object is much less .

For example, to lift a box weighing 150 N, one would need to exert 150 N of force without the help of pulleys.

However, by using just two pulleys, the person would only need to use 50 N of force.

 

A combined pulley makes life easier as the effort needed to lift the load is less than  half the weight of the load.

The main advantage of this pulley is that the amount of effort is less than half of the load.

The main disadvantage is it travels a very long distance.

source: http://www.the-office.com