A differential directs the torque of an engine to spin the wheels of a vehicle. The differential also helps the gears reduce the revolutions per minute of the engine to the various speeds at which each of the wheels is required to rotate. The wheels need to rotate at different speeds when a vehicle turns. The front and rear wheels have different distances to travel in the same time. This also applies to the wheels in the direction of the turn as opposed to the wheels on the far side. The free wheels can do this but the differential is needed for the 2 wheels on the drive as they both get equal power from the same source.There are many kinds of differentials. The simplest and most common is known as an open differential. It has a set of small gears called pinions and a ring gear in a housing. The input pinion or gear turns the ring gear, while the other pinions are stationery, when a vehicle with an open differential moves forward in a straight line. Both wheels on the drive rotate at the same speed while the other 2 wheels that are not connected to the engine and transmission, free wheel. The set of pinions make the wheel on the drive and in the direction of a turn, move slower than the wheel on the far side, when the driver wishes to turn and change direction. The 2 wheels that are not connected to the drive continue to freewheel. This makes the vehicle turn in a particular direction. The open differential, in this manner, enables a vehicle to either move in a straight direction, or to make a turn.The open differential applies equal torque or turning power to both wheels on the drive. This torque has to be commensurate with the traction or grip that the driven surface provides. The wheels will slip and the vehicle will not move if torque is more than traction. A vehicle in a situation in which one wheel on the drive is on a surface with greater traction than the other, will be stuck, as the wheel to which the differential applies too much torque, will slip. This shortcoming of an open differential is overcome by a limited slip differential.A limited slip differential is an advance on an open differential. A limited slip differential can apply varying levels of torque to either wheel, while an open differential can only apply equal turning force on both wheels on the drive. There are many kinds of limited slip differentials. A clutch limited slip differential has a set of springs and clutches. The springs fix the clutches against the housing of the differential and offer resistance to the gear system that applies different levels of torque to each of the wheels on the drive. This enables a vehicle to continue to move, albeit with reduced power when one driven wheel has better traction than the other, which may have a tendency to slip. Viscous coupling is another kind of limited slip differential used on all-wheel-drive vehicles. This kind differential can distribute or direct torque between all four wheels, depending upon which ones have traction. The Torsen differential is a variant of an all wheel differential, used for specialized application such as movement at high speed on surfaces with varying and low traction.A locking differential uses electric, pneumatic or hydraulic power to bind output gears or pinions in the housing. This enables wheels to rotate at the same speed even when on surfaces of varying and different traction. Locking differentials are used on vehicles that are used on rough or wild terrain without paved roads.Vehicles without differentials would be difficult to turn and would cause great strain and wear on various parts of the transmission and the axles. Such vehicles would also become immobile on surface such as ice that offer very little traction or grip. The differential makes for easier turning, reduces strain on the transmission and enhances the performance of a vehicle in off-road situations. News source: Car Pros