On a helicopter the tail rotor is very important. When you spin up a helicopter, the blades will rotate, but the helicopter body will tend to rotate in the opposite direction to the rotor. This is called ‘torque reaction’. So, if the blades rotate clockwise the body will rotate counter-clockwise.

Newton's third law of motion states, "To every action there is an equal and opposite reaction". A tail rotor is used to compensate for this torque and hold the helicopter straight. On a co-axial like the like the helicopters made by the Kamov Company (and the Lama RC helicopter) the main blades spin in opposite directions, so their reactions cancel each other, hence there is no need for a tail rotor.

Below is the KA-50 helicopter, notice there is no need for a tail rotor. The counter rotating blades are cancelling each other out and allowing the helicopter to hover perfectly. This design has been proven to be a good idea for small landing sites as you can shorten the tail and reduce the overall length of the helicopter.