4WD = Low range and High Range gearing
AWD = Only high range gearing
Part-time 4WD: No centre differential.
Permanent 4WD:. No two wheel drive mode. System is equipped with a centre differential, and hence is safe to use on all surfaces. All four wheels are powered all of the time (usually 50/50 front and rear axles). This is arguably the best system since the torque split ratio does not change and is the most predictable. All wheels "help out" all of the time and this stabilises the vehicle + improves handling. : MB M-class SUV, the Range/Land Rovers.
Full-time 4WD: Basically permanent 4WD but with a 2WD mode. Jeep selct trak as fitted to cherokees
Permanent AWD: Basically permanent 4WD but without low range gearing. Examples include the Audi Quattro AWD system, the MB's 4-matic 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.
in a nutshell.