alt Hacker News^ this. 4wd is used on offroads and locks all wheels together, transfering equal power to all wheels. AWD varies power using different technologies, from a simple Haldex that transfers power back when front wheels slip to complex sensor controlled individual wheels. Some are front-biased (generally but not exclusively cheaper ones), some are rear-biased (mainly in sports cars) and some are true AWD, depending on the needs and expected pricing of the vehicle.
Not quite, no. It only locks all the wheels together if you also have axle lockers.
You might have something like a Series Landrover transfer box where in 4WD mode the front and rear axles are locked together and in 2WD mode only the rear axle is driven.
But does that make a Defender, RRC, or Disco with LT230 transfer box which has a lockable diff AWD or 4WD? The centre diff supplies power to both front and rear all the time, but can be locked in slippy conditions otherwise if one wheel spins *none* of the wheels get drive - the axle and centre diff will both spin.
Then what about the later Borg-Warner transfer box in later Range Rovers, where you still have a centre diff and epicyclic gearbox to set high and low range, but there's no diff lock - there's a massive viscous coupling that turns it into a ridiculously stiff LSD.