It's a blurry line between all-wheel drive and a true 4x4 with four-wheel drive, but we'll explain
As you're certainly aware, most cars these days have four wheels. So why do people make a distinction between all-wheel drive and four-wheel drive when driving four wheels clearly means you're driving all of them, and vice versa? Despite the confusing names, and some confusing historical usage, there is a meaningful difference between four-wheel drive (4WD or 4x4) and all-wheel drive (AWD) systems.
Before we delve into the specifics, it's important to understand the basics of each system. First up, what is four-wheel drive? Often written 4WD, but also represented as 4x4, a four-wheel-drive system does just what it says: Its drivetrain is capable of sending power to all four wheels. A 4WD system also typically has a transfer case, which the driver must control via a secondary shift knob or electronic selection dial, offering a low range for better gearing off the pavement and a high range for improved highway gearing. The transfer case, while indicative of a "true 4WD" system, is not, strictly speaking, mandatory to qualify as 4WD. Four-wheel drive systems can be full-time, meaning all four wheels are powered at all times, or part-time/on-demand, where power is mostly sent to one axle (almost always the rear with a 4WD vehicle) until the driver chooses to engage the other axle for four-wheel drive. Increasingly, some 4WD vehicles can do this automatically (4A or Auto modes).
All-wheel drive, or AWD systems, like their 4WD counterparts, are also able to supply engine power to all of the wheels. Likewise, an AWD system can be full-time or part-time/on-demand. However, very few rely on the driver to engage all-wheel-drive. It's most common for AWD vehicles to be powered by one axle (typically the front in this instance), until the vehicle senses or anticipates wheel slippage and sends power to the other axle accordingly. Those with full-time systems are also able to change the amount of power going to the front and rear (this is common among rear-wheel-drive based vehicles, most notably luxury and performance-oriented ones). Unlike a 4WD system, however, AWD systems typically do not have a transfer case that the driver can use to select from extended gearing ranges for on- and off-road use.
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Complicating matters, however, is that these descriptions of 4WD and AWD systems do not entirely line up with how the terms are used, especially by the marketing arms of vehicle and parts manufacturers. In practice, in the United States since the 1970s especially, the AWD term has indicated permanent drive sent to each wheel, but without the use of a transfer case. Four-wheel drive, on the other hand, has been used to refer to part-time or on-demand systems that do use a transfer case. Moreover, though the line has been blurred significantly in the past decade, AWD systems were historically marketed for vehicles intended primarily for on-road use, while the 4WD systems were marketed with vehicles intended for substantial off-road use.
