Pulleys

How do pulleys work?
To understand how pulleys work, we must first understand the equation 'Work equals distance multiplied by force'. When it comes to pulleys, the work is the amount of energy required to move the object, the distance is how far the object is moved, and the force would be the mass of the object multiplied by the Earth's gravity. Work will always equal the product of distance and force. That means, if the distance is increased, the force will have to decrease to compensate. When using pulleys, the weight of the object is supported by multiple ropes. This means that all the ropes supporting the object will have to be moved further but the force that is supported by the ropes will decrease.

Some other stuff:
Force = mass x acceleration
The mass of the object is measured in kg and the acceleration is 10ms⁻²  due to gravity. The force is measured in Newtons.
Work = Force x distance
The force is measured using the equation above and the distance is how far the object is moved. It is usually measured in meters. Work is measured in Joules.
The mechanical advantage of using pulleys
The amount of force will be divided between all the ropes that are connected to the object.
The amount of force on one rope will be equivalent to the other rope.

In this photo, the force required to lift the object is 100N (Newtons).

If you were to move the object up 4 meters, it would take 400 Joules of energy.
Both point A and B are being pulled by 100N of force.

Normally, It would require 100N of force to lift this object, but because it is supported by 2 ropes, the force is divided. And because point B has 50N on it, point A would also have to have 50N to balance it out. 

The amount of work required to lift the object up 4 meters is still 400 joules of energy. You might ask why that is since it only takes 50N of force. Well, this is because, in order to move the object up by 4 meters, the length of both rope B and C must decrease. In order for that to happen, Point A must be pulled twice as far.
Thus resulting in the same amount of work. The main point of using a pulley in not to decrease the work, but to make it easier. This is done simply by decreasing the force required to move the object, as well as increasing the distance in which you have to pull on the rope.