The terms "all-wheel drive" and "four-wheel drive" are often used interchangeably, but they really shouldn't be. There are key differences between the two. Perhaps you've never known the differences between all- and four-wheel drive and were afraid to ask, or maybe you didn't realize they mean different things. This video from Engineering Explained sums it up succinctly, just for you.
Basically, an all-wheel drive system can send a variable amount of torque to each axle, and the driver doesn't typically control this. A four-wheel drive system sends a fixed amount of power to each axle, and it can be switched on or off by the driver. So, how do they go about doing their jobs? Generally, an all-wheel drive system uses a center differential to distribute the engine's torque between the two axles, while four-wheel drive relies on a transfer case, which functions like a locked differential.
At this point you might be wondering why there's a need for both of these types of systems, since the aim of both is to distribute power to all four wheels. Four-wheel drive is great for off-roading and other low-traction scenarios, since the system sends a fixed amount of power to each tire. Whichever tire has the most traction is guaranteed to get the power it needs, helping prevent the vehicle from getting stuck.
But four-wheel drive often doesn't work on the road for the exact same reason it works so well off road. In a corner, the wheels of a car spin at different speeds. With four-wheel drive selected, the system tries to get each wheel spinning at similar speeds, which makes on-road cornering difficult. That's why most four-wheel drive systems can be operated in a two-wheel drive mode, which is recommended for road use.
All-wheel drive systems are great for road usage because they can actively send power to the wheel (or wheels) that need it most. Some all-wheel drive systems have a fixed torque split between the front and rear axles, but those don't have the same issues with cornering as four-wheel drive systems since they rely on a differential rather than a transfer case.
Of course, there are lots of different types of all- and four-wheel drive systems that buck these conventions. Engineering Explained uses the new BMW M5 as an example, which has a four-wheel drive system that uses clever hardware to behave more like an all-wheel drive system. And pictured at the top of this post is , which has a system that combines four- and all-wheel drive.