4WD = Low range and High Range gearing

4WD = Low range and High Range gearing
AWD = Only high range gearing

Permanent AWD: Basically permanent 4WD but without low range gearing. Examples include the Audi Quattro AWD system, the MB's 4-matic AWD system, Subaru's manual transmission AWD system.

Full-time AWD: System is active at all times, however in most cases, the one set of wheels (usually the rears) only receive 5-10% of the engine's power unless slippage occurs. At that point, power is progressively transfered to the opposite axle to help out. Some systems can transfer power to the rear upon acceleration to improve traction. However, they revert to 2WD mode when coasting.