This is a nice simplified drawing of a Horizontal Axis Wind Turbine that is using upwind construction. The rotor assembly is ahead of the turbine in the wind stream. One complication of this design, is that a yaw drive, or tail is required to keep these unstable designs pointed into the wind. This unit has a yaw drive (numbers 13/14) they take info from the wind vein which turns motors and actuators. The large gear is affixed to the tower, and the smaller gears are firmly mounted on the nacelle. Many of the most commonly installed units today are upwind, steel tube constructed.