Our next simple machine is a pulley (here by Evilnurn). A simple pulley changes the direction of a force transmitted by a cable as that cable moves around the circumference of the pulley. For instance, you could attach a rope to a rock and loop the rope over a pulley above you. If you pull down with a force of ten pounds, the result will be an upward force of ten pounds on that rock.
If you loop back and forth between two or more pulleys, you create a block and tackle (here by Louise Dade). This gives you a mechanical advantage. For instance, if there are two parallel stretches of rope, as in the model below, if you apply ten pounds of force in pulling the rope, twenty pounds of force will be exerted to lift the load. BTW, you never get something for nothing - you have to pull the rope twice as far to get double the force.