Main Rotor System
The main rotor system is where all the pilot input in the cyclic and collective control gets turned into flight direction, whether it is up or down, left or right, forward and backwards and even a combination of all 6 directions, the main rotor system is what the helicopter is flying on.
The rotor blades turn the rotating force into lift based off of the angle of the blades in relation to their flight path. By changing the angle of the blades via the swash plate, cyclic and collective input, different amounts of lift can be generated at different angles along the circumference of the rotor disk. These differing amounts of lift is what causes the helicopter to go left and right, forwards and backwards. The collective control changes all blade angles equally in order to allow the up and down motion in flight. The cyclic and collective work together and their input is cumulative. This means that if the pilot is turning right and he or she picks up on the collective, he or she will still be turning right, but also gaining altitude.
A unique problem inherent to all helicopter design is dissymmetry of lift. What this means is that outside of hover, the lift created by the leading blade is different from the lift created by the retreating blade. This difference in lift causes the rotor disk to tilt, the greater the airspeed, the greater the effect of the dissymmetry of lift. Dissymmetry of lift also defines the maximum airspeed for the helicopter. You can only fly so fast before the retreating blade stalls completely. The problems caused by this phenomenon prevented any practical use of helicopters until they could be overcome. In this example, the rotor hub and blades are semi-rigid and are allowed to tilt. This tilting action allows the rotor disk to move independently from the helicopter negating the effects until the limit of tilt is reached. However the aircraft forward airspeed is limited in order to prevent this from happening. Other designs utilize more rotor blades and spin them at a lower rpm while allowing each blade to move independently. This is known as a fully articulated system. Because there are more blades and lower rpm's, retreating blade stall is delayed allowing for faster airspeeds to be reached. Other designs include co-axial rotor systems which negate retreating blade stall almost all together and do not require the use of a tail rotor system, though this design is much more complex and hasn't seen much widespread use outside of eastern helicopter designs from Russia. There is also the rigid designs which look much like a propeller. However this design is not in common use. Tandem rotor systems utilize one of the three, rigid, semi-rigid or articulated hub designs, however instead of mounting them on top of each other as in a co-axial system, they mount two separate rotor hubs. This design can be seen in the Chinook helicopter or K-max. A good website to visit to learn more on rotor hub design is http://www.aviastar.org/theory/rotor.html.