With a conventional helicopter, the tail rotor stops the helicopter spinning around the mast as explained by Fareastdriver. So the tail rotor keeps the helicopter pointed in the right direction without contributing to the overall lifting capability of the helicopter. However it does require engine torque to be keep turning and this takes away power from the main rotor. In certain conditions of wind etc, the tail rotor can be absorbing up to 18% engine torque - without producing vertical lift. A Chinook-type rotor system does not need a tail rotor therefore all engine torque is directed to the main rotors producing lift, a far more efficient result. The downside of a Chinook is that it has two main transmissions, two main rotor heads and two sets of main rotor blades and this, in turn, means that although the efficiency is increased they're bloody expensive to buy, maintain and operate - which is why, generally speaking, it is mainly military operators who have them.