This is the differential mechanism. The mechanism is to transmit power on the two power wheels. The spider gear moves along with the axis of the ring gear, when the vehicle is desired to turn left the side, the spider gear will start rotating in anti clock wise direction increasing the number of rotation of the right wheel. When its desired to turn right the spider gear will rotate in clock wise direction increasing the number of rotation of the left wheel. This helps to avoid slipping of the wheels and make a proper split of power at turns. 

Great article

i made a same article on lathe machine operations

THIS IS THE DIFFERENTIAL MECHANISIM GEAR SYSTEM