This is a copy paste thing, and the general idea of a diff. (I not great at explaining things)

The differential provides the means for allowing the outside wheel to spin faster than the inside wheel on corners or curves. The two side gears are splined to the axle shafts, while the two pinion (spider) gears act as idlers to transfer the power from the carrier to the side gears.
The pinion (spider) gears also balance the power load between the side gears while allowing unequal axle speeds when the vehicle is turning.