From Edmunds;
What Is All-Wheel Drive?
As the name implies, all-wheel-drive systems power both the front and rear wheels all the time. But in practice, there are actually two types of drivetrains that are called AWD. One does, in fact, drive all the wheels continuously, and some manufacturers refer to this as full-time AWD. The second, often called part-time AWD or automatic AWD, operates most of the time in two-wheel-drive mode, with power delivered to all four corners only when additional traction control is needed.
How Does All-Wheel Drive Work?
AWD systems, both full-time and part-time, generally operate with no input from the driver, although some offer selectable modes that allow a degree of control over how much power goes where. All the wheels get torque through a series of differentials, viscous couplings and/or multiplate clutches, which help distribute power to the wheels so that the car's traction is optimized. The vehicle still operates smoothly under normal conditions.
Full-Time AWD
In full-time AWD, both the front and rear axles are driven all the time. On dry pavement, this kind of AWD can help the vehicle handle better and ensure that full power gets to the road. And in slippery conditions, such as ice, snow or mud, it provides additional traction for safer, more confident handling.
Part-Time AWD
In normal operation, part-time AWD sends torque to two driven wheels, either the front or rear, depending on the make and model. The system then automatically engages the other two wheels when road conditions demand extra traction. Modern part-time AWD uses an array of electronic sensors that feed information to a computer, which controls the amount of power directed to each wheel.
All-Wheel-Drive Pros and Cons
The best thing about AWD is that the driver doesn't have to make any decisions about engaging the system. Either all the wheels are being driven full time, or the system itself is designed to sense loss of traction and send power where it's needed. AWD is available on a wide variety of vehicles, from compact sedans to performance models to all sizes of SUVs, giving you a broad range of choices.
While AWD is able to work well in a variety of conditions, from rain to snow to light off-roading, it's generally considered a lesser choice by serious off-roaders. This perception is changing somewhat as modern AWD systems get better and more capable, but many drivers who like to venture far off the beaten path still prefer to decide for themselves when to engage four-wheel drive. AWD also increases the cost of a vehicle and, in most cases, will reduce fuel economy.