Why a helicopter has a second rotor on its tail?

When a helicopters main blades rotate it causes a reciprocal rotation in the body of the helicopter, a torque reaction - basically it means that the helicopter tries to rotate in a direction opposite to the rotation of the blades (newtons 3rd law). To prevent thsi there are two way 1) have another set of blades that rotate in the opposite direction like a Chinook helicopter where one set rotates anticlockwise and the other rotates clockwise or 2) Use a tail roter to counteract the force of the rotation.

You must have seen in failms wwhere the tail rotor gets snagged or shoot up and then the helicopter starts spinning madly.. well, that's what happens when the helicopter loses the ability to control the torque reaction of the main blade set.

The tail rotor is also there to control the direction of flight in the horizontal which it does by altering the pitch of the blades in the tail rotor as well as, or alternatively, by increasing or decreasing the rate of rotation in the rotor to over or under compensate for the torque reaction.