There's a lot of blurring and different labeling of 4WD systems based on the market segment.
Generally it's more helpful to think of 4WD/AWD as fitting into three categories:
1: A system with a transfer case that predominantly drives one axle (almost always the rear, and almost always labeled "4WD"). This is your typical pickup truck 4wd system. When it is engaged, the two axles are locked together and may as well be welded and they'll turn at the same speed.
2: A system with a center differential, in which both axles are driven but may turn at different speeds (Almost always labeled AWD). The differential may allow full or progressive locking and/or torque vectoring. It's rare to find one of these that can fully lock (Subaru WRX STI is the only one I can think of), but it's common to see ones that favor one axle (Audi, BMW, Mercedes) or that can partially lock in response to slipping of one axle (often with the brand name Torsen). These are almost always found on passenger cars, and with few exceptions they're found on cars with longitudinally mounted engines (Mitsubishi EVO being the only transverse layout car with a center diff I can think of), that means they're usually found on higher end cars that are based on front-engine rear-wheel drive models.
3: A system with a power-take-off unit (often brand name Haldex), in which one axle is driven and a clutch pack can partially or fully engage the other axle. (this is often labeled AWD when put on a passenger car, and 4WD when placed on a "truck") The axles can operate at different speeds but can also fully lock in the way a transfer case can. Typically you find these on "AWD" cars that are based on transverse-mounted front-drive models as it's easier to add such a system and get partial engagement of the rear axle, but you'll also find them on performance AWD applications based on rear-drive mid/rear-engine models like Porsche 911s or the Buggatti Veyron. You'll even find it labeled as a 4WD system on "trucks" like Honda Ridgeline or Pilot, or on the new Ford Raptor. These tend to operate more like an automatic transfer case, as the only way to partially engage the secondary axle is to slip the clutch, which you don't want to do 100% of the time. (Ford actually calls this system a transfer case on the Raptor).
You'll see all sorts of spurious claims about systems "sending 100% of the torque to the rear axle" (often just means that the system can fully lock, and 100% torque to the rears means the fronts are in the air). You'll also see things like "rear biased", which sometimes means you have a center differential that overdrives the rear axle by default, which is great, but is often misleadingly applied to PTO/Haldex systems where the rears are also overdriven, but only during the rare times when the system is engaged.
Basically if you're looking for traction in low speed settings where you might not have grip at all on one axle, like offroading or being stuck in the snow, you want as many of your axles locked as possible, which technically you can get with any of these systems, but is usually seen on transfer cases or haldex/PTO systems.
If you're driving quickly on pavement with some traction, you usually want something more like a center differential, preferably one that can partially or entirely lock when you lose some grip at one end.
Right. I've got a Golf R with Haldex; it allows 30+mpg under cruising conditions but can "lock 100%" in the sense that the fronts and rears are locked together. It's created a lot of forum wars because some people read the marketing materials saying "100% of torque to the rear wheels" or "max 50/50 split" and don't understand the difference. If it's fully locked, on dry pavement, of course you're getting 50/50. If the front wheels are on a magic 0 friction surface, of course, all of your torque is going to the rears because the fronts have nothing to grip on, but they're all spinning at the same speed (theoretically).
Generally it's more helpful to think of 4WD/AWD as fitting into three categories:
1: A system with a transfer case that predominantly drives one axle (almost always the rear, and almost always labeled "4WD"). This is your typical pickup truck 4wd system. When it is engaged, the two axles are locked together and may as well be welded and they'll turn at the same speed.
2: A system with a center differential, in which both axles are driven but may turn at different speeds (Almost always labeled AWD). The differential may allow full or progressive locking and/or torque vectoring. It's rare to find one of these that can fully lock (Subaru WRX STI is the only one I can think of), but it's common to see ones that favor one axle (Audi, BMW, Mercedes) or that can partially lock in response to slipping of one axle (often with the brand name Torsen). These are almost always found on passenger cars, and with few exceptions they're found on cars with longitudinally mounted engines (Mitsubishi EVO being the only transverse layout car with a center diff I can think of), that means they're usually found on higher end cars that are based on front-engine rear-wheel drive models.
3: A system with a power-take-off unit (often brand name Haldex), in which one axle is driven and a clutch pack can partially or fully engage the other axle. (this is often labeled AWD when put on a passenger car, and 4WD when placed on a "truck") The axles can operate at different speeds but can also fully lock in the way a transfer case can. Typically you find these on "AWD" cars that are based on transverse-mounted front-drive models as it's easier to add such a system and get partial engagement of the rear axle, but you'll also find them on performance AWD applications based on rear-drive mid/rear-engine models like Porsche 911s or the Buggatti Veyron. You'll even find it labeled as a 4WD system on "trucks" like Honda Ridgeline or Pilot, or on the new Ford Raptor. These tend to operate more like an automatic transfer case, as the only way to partially engage the secondary axle is to slip the clutch, which you don't want to do 100% of the time. (Ford actually calls this system a transfer case on the Raptor).
You'll see all sorts of spurious claims about systems "sending 100% of the torque to the rear axle" (often just means that the system can fully lock, and 100% torque to the rears means the fronts are in the air). You'll also see things like "rear biased", which sometimes means you have a center differential that overdrives the rear axle by default, which is great, but is often misleadingly applied to PTO/Haldex systems where the rears are also overdriven, but only during the rare times when the system is engaged.
Basically if you're looking for traction in low speed settings where you might not have grip at all on one axle, like offroading or being stuck in the snow, you want as many of your axles locked as possible, which technically you can get with any of these systems, but is usually seen on transfer cases or haldex/PTO systems.
If you're driving quickly on pavement with some traction, you usually want something more like a center differential, preferably one that can partially or entirely lock when you lose some grip at one end.