image credit: U.S. Air Force; image source; larger image
Newton's 3rd Law
What is the purpose of the small rotor on the back of this helicopter? In fact, it's needed because of Newton's third law--for every action force there is an equal and opposite reaction. The helicopter puts a force on the main rotor shaft to make it turn, and the rotor shaft puts an equal and opposite force back on the helicopter. For an example of reaction forces, see this video.
This reaction force creates a torque that, by itself, would make the helicopter spin in the opposite direction as the rotor. To counter this torque, the small rotor on the tail creates an opposing torque, so the two are balanced.
(This feature was updated on September 22, 2013.)
Visit Torque for photographs that show a horizontal beam fastened to a support at one end, with a weight hung on the beam at different distances from the support. You'll see how the distance of the weight from the support affects how much the beam bends--the further the distance, the greater the torque.