system
Given the following post, respond as if you're commenting on Reddit:
human
(Eli5) what's the difference between all wheel drive, symmetrical all wheel drive (remember this from a commercial, Subaru I think) and 4 wheel drive?
gpt
First 4WD vs AWD.

4WD is generally used in Trucks and SUVs and is generally not an "all the time" thing, it is just engaged when it is necessary. What takes place is a mechanical locking of the differentials to ensure that all 4 wheels rotate "as one". what this means is all 4 wheels are rotating the same amount which aids significantly in loss of traction scenarios, but you definitely DON'T WANT during daily driving. Wheels need to be able to rotate at individual speeds especially when making turns otherwise tires will get destroyed at best, or at worst you'll end up in an accident or end up destroying the vehicle's drivetrain. 

AWD utilizes limited slip differentials to enable the engine to drive all 4 wheels at once, which is a happy medium for cars that often find themselves in icy situations. LSDs allow the wheels to rotate at different speeds.

now what Subaru offers that others don't: symmetry. 

The vast majority of a manufacturers lineup is front wheel drive vehicles and then for a premium, you can add AWD functionality. What does this mean? They take their front wheel drive car and slap on a drive shaft and an extra differential and tada, AWD. 

Subaru only makes AWD vehicles (except for the BRZ which is RWD only) and therefore they make the vehicle with the purpose of being AWD from the beginning. 

Now in the average vehicle, you have a transversely mounted engine which means the power from the engine comes out towards the driver's side front wheel, That power then needs to turn 90 degrees back towards the rear of the car, 90 degrees again towards the passenger's side, then once in the center, 90 degrees again towards the rear of the car. Every time you have to change the direction of the power, you lose power and create lag. Subaru's use of a boxer engine allows them to not need to change power direction at all before heading towards the rear of the car... therefore the entire drive train and by consequence power output is completely symmetrical.


What does this actually buy you? Well Subaru claims a better AWD system; at any rate it certainly buys you more of your engine's power making it to the wheels and slightly quicker.