Four Wheel Drive

Four-wheel drive, 4×4 ("four by four"), and 4WD, is a form of drivetrain capable of providing power to all wheel ends of a two-axled vehicle simultaneously. It may be full-time, or on-demand, and is typically linked via a transfer case which provides an additional output drive-shaft, along with additional gear ranges.

A four-wheeled vehicle with power supplied to both axles may sometimes be described as "all-wheel drive" (AWD). However, "four-wheel drive" typically refers to a set of specific components and functions, and/or intended offroad application, which generally complies with modern use of the terminology.

4×4/4WD/AWD systems were developed in many different markets and used in many different vehicle platforms. There is no universally accepted set of terminology to describe the various architectures and functions. The terms used by various manufactures often reflect marketing rather than engineering considerations or significant technical differences between systems.

Four-by-four (4×4) refers to the general class of vehicles. The first figure represents the total wheels (more precisely, axle ends), and the second, the number that are powered. Syntactically, 4×2 means a four-wheel vehicle that transmits engine power to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive. Alternatively, a 6×4 vehicle has three axles, two of which provide power to two wheel ends each.

Four wheel drive (4WD) refers to vehicles with two axles providing power to four wheel ends. In the North American market the term generally refers to a system that is optimized for off-road driving conditions. The term "4WD" is typically designated for vehicles equipped with a transfercase which switches between 2WD and 4WD operating modes, either manually or automatically.

All wheel drive (AWD) historically was synonymous with "four-wheel drive" on four-wheeled vehicles, and six-wheel drive on 6×6s, and so on, being used in that fashion at least as early as the 1920s. Today in North America the term is applied to both heavy vehicles as well as light passenger vehicles. When referring to heavy vehicles the term is increasingly applied to mean "permanent multiple-wheel drive" on 2×2, 4×4, 6×6 or 8×8 drive train systems that include a differential between the front and rear drive shafts. This is often coupled with some sort of anti-slip technology, increasingly hydraulic-based, that allows differentials to spin at different speeds but still be capable of transferring torque from a wheel with poor traction to one with better. Typical AWD systems work well on all surfaces, but are not intended for more extreme off-road use. When used to describe AWD systems in light passenger vehicles it describes a system that either applies power to all four wheels (permanently or on demand) and targeted as improving on road traction and performance, particularly in inclement conditions, rather than for off road applications.

...
Wikipedia