Torque
According to Isaac Newton, an object at rest will remain at rest, and an object will remain in motion unless acted upon by an outside force. A force, therefore, is what causes any object to move. Any force that causes an object to rotate, turn, or twist is called a torque. Torque is equal to the amount of force being exerted on the object times the object's rotation point to the location where the force is being applied on the object.
Seesaws are a good example of torque. Many people have had the experience of a large person sitting on one end of the seesaw, and a small person on the other end. If the larger person is sitting closer to the pivot point of the seesaw, the smaller person can lift them with little or no problem.
The reason this is possible comes from the difference in torque experienced by each person. Even though the smaller person exerts a smaller force, his distance from the pivot point (lever arm) is longer, hence a large torque. The larger person exerts a larger force; however, because he is closer to the pivot point, his lever arm is shorter, hence a smaller torque.
Wrenches also work by torque. (Some wrenches are even calibrated to display the amount of torque you are applying to a nut; they are called torque wrenches.) The nut (or bolt) is the point of rotation because we either want to tighten or loosen it by having it turn. The force is being exerted by your hand and arm. Since we try to pull or push (exert a force) at right angles on the wrench's handle, the lever arm is then the length of the wrench's handle. To increase the torque on the nut, we must either increase how hard we pull or push on the lever arm; or, increase the length of the lever arm by placing a pipe over the end of the wrench.
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