4WD stands for Four Wheel Drive. It simply means that all for wheels are driven. It is also expressed as 4X4 sometimes. 4X4 mean the 4 out of 4 wheels are being driven. The would also work in a 6X6 where all 6 wheels are being driven on a 6 wheeled vehicle. 4WD and 4X4 do differ from AWD (All-Wheel drive) in that they are part time. The vehicle is normally driven in 2 wheel drive mode until more traction is needed then it can be manually or automatically shifted into 4 wheel drive. The reason for this is because 4 wheel drive vehicles lack a center differential. So while in 4 wheel drive the front and rear tires must spin the same speed. This causes binding and added stress to the drivetrain because while turning the front wheels will travel a further distance than the rear wheels. In contrast All-wheel drive vehicles do have a center differential and drive all 4 wheels all the time.
