system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5; The difference between 4WD and AWD?
gpt
1 diff = 2WD, either FWD or RWD depending on axle driven.

2 diff, one on each axle = 4WD, a selectable transfer case in the middle

3 diff, 3rd diff being the center diff splitting power fore and aft = AWD.

It's really as simple as that. A cars powered wheels must be allowed to rotate independantly of each other in order to navigate turns without binding or skipping; therefore a diff is used to allow this. For the same reason, if both axles are powered, each axle must be allowed to rotate independantly. A permanently engaged center diff allowes this, thus creating AWD.

A selectable transfer case does not, each wheel on each axle will, if the diffs are unlocked, be able to rotate independantly of the other wheel on the axle, but the two axles together will have the same average speed, which doesn't provide enough independance for use on high-grip surfaces like asphalt. However, forcing both axles to rotate at the same speed means that even if one axle has a total loss of traction, as long as the other one has traction, the vehicle will move forward.
This is why 4WD is superior to AWD off-road, but inferior on-road.

Superior to that however, is 4WD with locked diffs front a rear, in such a case, all wheels are forced to rotate at the same speed regardless of differences in traction, so even if 3 wheels are slipping, the 4th, if it has traction, will still get power and still push the vehicle forward; making this setup unbeatable off-road; and unusable on road.

So, you might have realized by now that AWD is actually pretty limited in its most simple form, if all diff's are open and any wheel lacks traction, the vehicle will tend to simply spin that one slipping wheel. For this reason, most, if not all, AWD systems are paired with either limited slip diff's, which can be mechanical or electronically controlled, or traction control systems that apply the brakes to individual wheels and limit throttle input in order to ensure as best as possible that wheels with traction receive enough power to drive the vehicle forward and help maintain control. In serious off-road conditions they still can not match true 4WD with lockers, however.