There are so many varieties of AWD. Most are wet-clutched (inside or outside of the main transmission), some are lockable or torsen center differentials, Prius adds electric power to the rear wheels to complement the FWD hybrid setup. Traditional 4WD with a transfer case using a manual shifter-actuated gear selector isn't very common any more. My 1999 Suburban had a wet clutch in a standard truck-shaped transfer case, one side of the front differential had a solenoid to lock/unlock one wheel to the side gear to keep the front drive shaft from spinning in RWD mode, and used a motor to mechanically engage or disengage the wet clutch (between the front and rear outputs) and to slide the engagement ring to offer AWD (rear-wheel biased, engaged when front and rear wheel speeds differed anywhere from 0 to 100% torque transfer) or 4WD (clutch fully engaged), and even 4WD-LOW by running the motor the other direction to engage the planetary gearing with the rear drive shaft.

In my mind, the biggest difference is whether front and rear drive shafts turn at exactly the same rate; if so it's "4WD". If clutch slippage or a differential allows different front and rear axle speeds then it's some form of AWD. But many AWD systems have clutches capable of effectively locking the front and rear driveshafts. E.g. the Suburban had tire-hop turning on pavement in 4WD mode which is about the most torque that drive-train would be expected to encounter.