[PHAEDRUS]

OK
here goes with my understanding of the subject,
1 AWD. it will send power to the fore and aft differentials through a series of spider gears or viscous and semi viscous couplers. only one range of power ie no lower gears and no transfer case per se. just sending power to from and rear diffs. variant is with the viscous couplers wich can enable torque splitting at a specified range to the front or rears of the vehicle. ie 50/50 or as low as 80/20
2. 4wd,
vehicle with a selectable transfer case that will give you the option of only engaging the rear drive line or the front and rear drive lines. also will usually have a low range gear set in a transfer case for lower gears in 4wd mode. slight variant being twin sticks wich can send power to front or rears alone or both. these will usually not have spider or viscous couplers between front and rear drive lines and will send power to the frotn and rear at a 50/50 basis, usually no variation on torque split but not sure on this one.
3. full time 4wd.
similar to the set up of the 4wd but using a series of spider gears or a viscous coupler to enable easier on road handling of the veh. the difference between this and awd is the addition of a transfer case with an additional lower range gear set.
holy chit that was long winded but I think kind of clarifies the issue???
Dave