Full time 4WD, also called permanent 4WD, is a system that powers all four wheels at all times and can be used full time on all surfaces including pavement. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.
2WD is no longer available. Each tire creates about 25% of the available torque when the ground is level with a consistant surface. Driver has a choice of a "4-high" (that's your every day setting) and "4-low".
Full time 4WD vehicles work very well on-road and are very capable off-road.
All wheel drive (AWD) is almost the same thing as full time 4WD - it is a system that powers all four wheels of a vehicle at all times as well. Full time symmetric AWD would be the best term to be used. Difference to full time 4WD is that a "4-low" setting is not available in AWD cars. Due to the lack of "low range" AWD vehicles are much less capable in off-road settings than full time 4WD vehicles, but work perfectly well on-road.
Automatic AWD system is the newest kid on the block. PR agnecy generated names like "Real Time 4WD", "intelligent AWD" or "active AWD" are hiding the fact that automatic AWD is essentially a sophisticated 2WD system. Automatic asymmetric AWD would be the best term for them.
Here is how they work:
Under normal conditions one axle gets 100% of the torque - meaning you are driving in 2WD. During traction loss at the driven axle (could be front or rear) a fully automatic system (hydraulic, mechanical or electronic) makes up to 50% of the torque to the axle with traction available. This means you have to lose traction in 2WD on your driven axle first and then the other axle will be added and try to keep the car moving and stable. Once the primary driven axle regains traction and both axles rotate at the same speed again, the system reverts back to 2WD. So, for a moment you had AWD.
Automatic asymmetric AWD is much less capable in off-road settings than full time AWD systems and inferior to full time 4WD. However, automatic asymmetrical AWD is becoming more and more sophisticated and offers pretty much everything consumers expect for everyday (pavement) driving.
Examples: Honda CRV, (newer) Toyota RAV4, LandRover Freelander, Isuzu Trooper (TOD), Volvo V70, 1999 and later Jeep Grand Cherokee (in high range).
Recently some magazines have called the automatic AWD system "part time 4WD", since it offers AWD only part of the time. They have a point - however, the term "part time 4WD" has been used since WW II for cars like the Willys and Jeep Wrangler and their part time 4WD . A manual system where the driver had to select 2WD or 4WD. The name coming from the fact that 4WD was designed to be used only part of the time (when off-road), most of the time it had to operated in 2WD (on-road).