All four wheels are driven by the engine. AWD systems are superior to 4WD as AWD can be used under any road conditions.
A vehicle drivetrain with a center differential having all four wheels under power at all times
Taken literally AWD means all four wheels can be driven, but there is some ambiguity as to just what "driven" means. If a centre diff' is used between the front and rear axles then all wheels share equal torque (unless one slips and an LSD or diff' lock takes over). A cheaper arrangement is to drive one axle via a viscous coupling or similar. In this case one axle receives no torque until the other axle slips - is this all wheel drive? Debatable.
all wheels provide driving force to enhance traction in inclement weather or when spirited driving is wanted. On-road all-wheel-drive systems are biased for sporty driving in clear to moderately inclement conditions. Four-Wheel-Drive or 4x4 systems are usually found on pickup trucks and SUVs and are capable of driving off-road in very challenging conditions.
Term used to describe a full time four wheel drive system or any other car drive system that continuously delivers power to all four wheels. This system is found primarily on station wagon cars and SUVs not utilized for significant off road use.
This is where all the wheels are utilized as active drive wheels for greater control in challenging road conditions. It works by dividing up the engine torque between the front and rear wheels in varying amounts depending on the needs. AWD is designed as an on-road control system and is therefore not the same as Four Wheel Drive (4WD or 4x4). (See also Torque).
