Pulleys are simple machines consisting of a suspended disk that allows the tension force in a rope to change direction. In a simple pulley configuration, the rope or cable runs from a suspended weight up to the pulley, then down to another, creating 2 lengths of rope or cable strands. However, the tension in both sections of rope is equal, even if both ends of the rope are being pulled by forces of different magnitudes.
Note: see common misconception no. 4