This is IMHO the best piece I have ever found on the web that without any hyperbole or bias outlines the differences between full time 4WD, part time 4WD, all-wheel-drive, locking differentials, open differentials, couplings and how all the pieces work together and the different pieces involved. It even includes a glossary. If you even remotely want to know how a 4-wheel drive system in any configuration works, the difference between 4WD and AWD, or what makes a system part-time versus full-time, I would call this REQUIRED reading.
Truck Trend 4-Wheel Drive Information Story
Enjoy!
Truck Trend 4-Wheel Drive Information Story
Enjoy!