All Wheel Drive

An all-wheel drive vehicle (AWD vehicle) is one with a powertrain capable of providing power to all its wheels, whether full-time or on-demand.

The most common forms of all-wheel drive are:

Vehicles may be either part-time all-wheel drive or full-time:

Today, the designation AWD is becoming narrowed, particularly in North America, to apply to vehicles with drive train systems that have permanent drive and usually include a differential between the front and rear drive shafts. However, the designations AWD and all-wheel drive long predated the trend, with Associated Equipment Company (AEC) building AWD trucks in conjunction with FWD (UK) in 1929, and General Motors manufacturing a line as "all-wheel drive" as early as the late 1930s. This distinction in terminology is not generally used outside North America.

When tyre grip is good during road driving a differential is used between the axles to avoid driveline windup. This is not required off-road, as the limited grip allows the tyres to slip relative to each other. All-wheel drive vehicles designed for extensive off-road use may not have such a differential and so they suffer from wind-up when used on-road. Selectable 4WD also avoids this problem and requires only a simple dog clutch in the transfer case, rather than a differential. For this reason, most early off-road vehicles used that system; e.g., Jeep, Land Rover.

As vehicles became more sophisticated and tyres gave better winter performance in the 1960s there was an interest in giving the benefits of all-wheel drive to conventional cars; not for off-road use but for winter use in snow or on wet roads. Esoteric vehicles such as the high-powered Jensen FF followed by the AMC Eagle, Subaru Leone and Audi Quattro series were the first to offer all-wheel drive in a high-speed road-based car. These, particularly the Quattro, would extensively develop this drivetrain with the use of viscous couplings and differentials to provide a safe and drivable car. The first off-road / on-road hybrids such as the Range Rover also chose the permanent all wheel drive system rather than manual selection.

...
Wikipedia