A quadcopter consists of four motors. Each motor produces thrust and torque. The quadcopter can hover and stay steady over the ground when all four motors have the same torque and the total thrust of the craft is equal to or greater than m*g, where m is the mass of the craft and g is the gravity constant. Motors one and four spin clockwise, as shown in the following figure:

Motors two and three, on the other hand, spin counter clockwise. Since every motor produces torque, motor one cancels out the torque produced from motor four and motor two cancels out the torque produced from motor three.

Let's have a look at the quadcopter's maneuvers. Assuming that CM is the center of the craft mass; in order to implement a forward movement, all that is needed is to lower the angular velocity of motor one and two. As a result, there will be less thrust produced in the front than the rear resulting in the quadcopter moving forward. Similarly, we can lower the angular velocity of motors one and three or three and four and have either left or right movement. Lastly, there is one more maneuver that a quadcopter can do. In order to have yaw rotation, we need to lower the speed of any of the two opposite motors. Actually, doing this will increase or decrease the total left or right torque. This creates an imbalance in the torque between the motors; thus, causing the quadcopter to have yaw rotation.