A tail rotor counteracts the torque produced by the main rotor.

Torque

Torque is essentially a moment (the product of the force and the distance from a pivot), but it is usually a term used when a force is being used to intentionally rotate something. Imagine that you are trying to unscrew the top of a jelly jar. The force exerted by your hand on the rim of the lid, times the distance from the rim to the center of the lid, is by definition the torque you are exerting on the lid. The torque is, like moments, equal to the force times the distance to the point of rotation. In aeronautical use, when we are considering the aerodynamic forces acting on a flight vehicle, we usually use the term "moments" rather than "torque."

An object may be acted upon by multiple torques. The motion of the system then depends on the net torque on the system. If a system is in equilibrium, or balanced, the torques would be equal and no net torque would act on the system.

The term "torque" is often used to describe the motion produced by helicopter rotors. Without a balancing rotor, the torque of a rotor would cause the helicopter to turn in an uncontrolled fashion. Helicopters often use tail rotors to counter the torque produced by the main rotor. Sometimes, helicopters use tandem rotors or coaxial rotors that moving in opposite directions.