A pulley is a wheel with a groove along its edge, for holding a rope or cable. A first-class pulley has a fixed axle and is used to redirect the rope (called a belt when it goes in a full circle). A second-class pulley has a free axle, and is used to transform forces - when stationary the total force on the axle balances the total force provided by the tension in the rope (which is constant in magnitude in each segment). So, for instance, if one end of the rope is attached to a fixed object, pulling on the other end will apply a doubled force to any object attached to the axle. This can be increased by using a block and tackle, where there are several pulleys on each axle. Pulleys are usually counted as one of the simple machines.
