Four-wheel drive, 4WD, or 4x4 ("four by four") is a four-wheeled vehicle with a drivetrain
that allows all four wheels to receive power from the engine simultaneously. In the United
States, these cars are 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 on many surfaces, and is an important part of rally racing on mostly-paved roads.

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. Whereas 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 receive different amounts of power. 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, while 4WD means only four of the wheels drive the vehicle.