Tail rotor

From Freepedia

The tail rotor of a helicopter is mounted on the tail of a single-rotor helicopter, perpendicular to the main rotor. It is primarily used in order to counteract the yaw motion and the torque that a rapidly turning disk naturally produces.

Most, if not all, dual-rotary helicopters do not use tail rotors, instead, the design of the two main rotors is such that they spin in the opposite directions of each other, thus each cancels out the torque and yaw produced by the other.