A pulley works by changing the direction that force must be applied to lift an object against gravity. Other, more complex, systems use multiple pulleys to reduce the amount of force that must be applied.
A simple pulley consists of a single wheel and a rope. Instead of needing to lift an object by picking it up off the ground, a pulley system allows the object to be lifted by merely pulling on the rope. Humans using pulley systems can use their own body weight to counteract the weight of the object, making lifting easier.
A more complex system, such as a block and tackle, makes lifting easier by using multiple pulleys. Usually, these pulleys consist of a rope wound around two axles multiple times. As the rope is pulled, the lower axle and the upper axle are slowly brought together. The number of times the rope is wound around each axle is what determines the rope's tension and, friction notwithstanding, the amount of force required to lift an object. However, energy is always conserved. Someone using a pulley system to use only 100 pounds of force to lift a 400-pound object must pull four times as much rope, expending the same amount of energy as it would take to lift a 100-pound object four times as far.