With the increased popularity and use of "all-wheel-drive" systems, buyers can easily be confused about some of the technologies referred to variously as AWD, 4WD, full-time, part-time, on-demand etc.
While different companies and marketing departments put their own spin on these systems, I am going to use conventions established by the Society of Automotive Engineers.
Recognizing that there was increasing confusion and misrepresentation, the all-wheel-drive standards committee of the SAE, representing a wide cross-section of auto makers and driveline suppliers, set out to provide industry-standard definitions of the various systems. The results were unanimously approved and published as an update to SAE Standard J1952 in December, 2005. (Thanks to Andy Perakes, a Ford engineer and driveline specialist, who was chairman of the SAE committee. Ironically and as an indication of how different companies use different terms, Perakes' title at Ford is 4-Wheel Drive/Driveline Technical Specialist.)
While there is little hope of getting car companies to abide by these definitions, the SAE says the only industry-defined term since 1991 when J1952 was first established, is AWD (All-Wheel-Drive), not 4WD.
The popular interpretation of AWD is that of an automatic, single-speed system that requires no driver intervention to engage. On the other hand, the popular conception of 4WD, also referred to as 4X4, is that of a two-speed system (high- and low-range) that incorporates a transfer case and requires the driver to enact engagement.
The SAE says "a conventional all-wheel-drive system consists of a means to distribute torque to all axles of a vehicle. Based on desired performance, traction and handling characteristics, there are different types of systems to achieve these ends. These all-wheel drive systems include 4X4, 6X6, etc." It breaks down the three basic types of systems as follows, but says combinations may also be used.
PART-TIME ALL-WHEEL DRIVE
These systems are what many people refer to as 4X4. They require the driver to move a lever, push a button or otherwise take action that cause a rigid coupling of the front and rear axles, in effect locking them together.
This commonly uses a transfer case, a device that allows a choice between high and low ranges, the latter meant for extreme conditions such as climbing or going down steep grades off road.
The gear multiplication of a transfer case makes low-range useful for very low speed operation only - but this is easily the most capable type of all-wheel-drive system.
FULL-TIME ALL-WHEEL DRIVE
The SAE says the front and rear axles of a full-time all-wheel-drive system are driven at all times through a centre differential.
In some newer, more sophisticated systems, very little power is sent to the second axle until sensors indicate a need.
This has become the most common system and the one where the marketing folks take more liberty. The SAE defines on-demand all-wheel-drive as a system where "the secondary drive axle is driven by an active or passive coupling device."
Most of the car-based SUVs and CUVs use this system coming with front-drive as standard equipment and all-wheel-drive as an option.
These systems normally send the vast majority of power to the front wheels, diverting some to the rear through a viscous coupling.
Each of these systems offers different levels of capability. And unfortunately, in their desire to make their latest car-based SUV or crossover appear more capable than it really is, some marketing departments take "liberties" with these definitions.