4WD systems include a (generally) manual shift transfer case that puts power to the front axle. In my case I also have manual locking front hubs (really old school here) The transfer case has three settings: 2WD (rear), 4WD high for driving in dirt and snow and ice, and 4WD low for when you need extra torque for steep hills or pulling. Because the power split is 50/50 front rear, handling can be a bit strange for the uninitiated.
In the AWD systems, you have a center differential along with the normal front & rear diffs. Some of them are electronically controlled to split the power asymmetrically and some are fixed. The asymmetric ones are more common. They provide excellent handling in pretty much all driving conditions and most drivers would not be able to tell if it was AWD at all.
The problem with the Subaru (and perhaps others) is that the tires need to be close to the same diameter on all four tires. Prolonged driving on an undersized spare can cause serious damage to the center differential. It is recommended to replace all tires at the same time. If you can not afford to, then 'shaving' the new tire down to match the older tires is recommended. I always rotated my tires through the spare so if I ever had a flat, the spare was always close enough to the driving tire diameter so that I had no worries. That being said, I only had one flat in the eight years that I owned the Forester. I have rolled the bead during rally races though.
It is also pretty much instant death to the center differential to tow the car on a car dolly or with a conventional tow truck. The safest way to tow a Subaru (and perhaps other AWD vehicles) is on a flat bed. It is sometimes permissible to tow the Subaru with ALL 4 wheels on the ground (I.E. behind a motor home), you should consult the owners manual for your specific vehicle - there are differences between auto and manual transmissions..