4WD in the united states is real 4WD where you have a true locking center differential, this is known as a "part time 4WD system" which can't be engaged on dry pavement or else you'll cause binding in the system. This type of system is found on any vehicle based on a truck (body on frame) platform, and some jeeps. The advantage is that it's actually capable of do real off roading because the diff is locked where gear teeth are actually locked together. So power goes to both front and rear axles no matter what, it's not right to even talk about power split because the front and rear shafts are essentially locked as one.
According to the SAE system, AWD is a type of 4WD system. AWD can typically be engaged on all surfaces and conditions, including dry pavement.
The rest of the world drives smaller vehicles, and very few trucks like ours. Nearly all of it is FWD based. So they have a wet clutch usually at the rear axle in which you send an electrical current to change the clamping force of the clutch. This wet clutch is designed to slip, but as it slips it also creates alot of heat. When you press the "LOCK" button on an AWD system, it doesn't actually lock anything, it just clamps the clutch at max force. But in offroad type conditions, the clutch will still slip and the car's computer will let go of the clutch to prevent overheating or damage. On many cars, the "lock" button will disengage after 18-25mph depending on the manufacturer. These systems work, but when trying to power out of something, they tend to disengage the rear axles, or allow too much slip of the center differential, not a true lock after too much torque is needed.
The advantage of using a center wet clutch is you can vary the torque ratio between front and rear at all times. This allows the system to be used on dry pavement because when you take a curve, the inside wheels travels less distance than the outside wheels, but they complete the turn in the same amount of time. Which means the outer wheels travel at a much higher speed than the inside wheels, so a true locking differential will cause binding or cause tires to slip which can be dangerous. But with a variable differential, it can allow for different wheel speeds and not hurt the system or cause instability.
I think this is one of the few things that American's are more progressive and correct about. I like the distinction between 4WD and AWD. The other is gasoline vs petrol, while both are correct. Gasoline is more specific in how the petroleum is processed.