Four-wheel drive, 4WD, or 4x4 ("four by four") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously. In the United States, these cars are often, but not always, included in the broader sport utility vehicle category. While many people associate the term with off-road vehicles, powering all four wheels provides better control in normal road cars on many surfaces, and is an important part of rally racing.
The term four-wheel drive describes truck-like vehicles that require the driver to manually switch between two-wheel drive mode for streets and four-wheel drive mode for low traction conditions such as ice, mud, snow, slippery surfaces, or loose gravel. All-wheel drive (AWD) is often used to describe a "full time" 4WD that may be used on dry pavement without destroying the drivetrain (It should be noted that "Full-Time" 4WD can be disengaged and the center differential can be locked, essentially turning it into regular 4WD. On the other hand, AWD cannot be disengaged and the center differential cannot be locked.) although the term may be abused when marketing a vehicle. AWD can be used on dry pavement because it employs the use of a center differential, which allows each tire to rotate at a different speed. This eliminates driveline binding, wheel hop and other driveline issues associated with the use of 4WD on dry pavement. With vehicles with more than four wheels, AWD means all wheels drive the vehicle, to varying degrees of engagement while 4WD means only four of the wheels drive the vehicle continuously. Identical drivetrain systems are commonly marketed under different names for upmarket and downmarket branding, and conversely different drivetrain systems are commonly marketed under the same name for brand uniformity. For example, Audi's quattro, DaimlerChrysler's 4Matic used on Mercedes-Benz products, BMW with the xDrive, and Volkswagen's 4motion can mean either an automatically-engaging system with a Haldex clutch or a continuously-operating system with a Torsen differential.