What does 4WD mean?

Often referred to as 4WD, four-by-four, 4x4, or all-wheel drive, four-wheel drive distributes power to all four wheels such that every wheel spins at the same constant rate. The system is most popularly adopted in vehicles designed and built for handling off-road conditions. 

4WD distributes power to all four wheels, but they will not necessarily be powered at the same time. Different 4WD systems include:

1) Full-time 4WD (or permanent 4WD): Power is constantly provided to all four wheels, usually with a power shift between the front and rear axles whenever necessary. Under Full-time 4WD, drivers can enjoy maximum traction regardless of driving conditions (dry or slippery). Additional action is not required for its activation.

2) Part-time 4WD: With part-time 4WD, drivers have to shift between the selectable 2WD and 4WD provided by the vehicle, usually with a lever or a switch - and part-time 4WD allows a driver to do this while driving. The lock between front and rear allow drivers better traction on slippery surfaces. While it is the best option for nearly all off-road conditions, driving with part-time 4WD on dry and/or smooth roads is not recommended.

3) Automatic 4WD: While the default is 2WD (front or rear), a full-time system will judge when 4WD or AWD is needed and automatically direct power to all four wheels - varying the distribution between the front and rear axles if needed. A slipping wheel is the most common activator of the shift, but a more sophisticated system will make the shift ahead of that slip.