Lucid AWD vs. 4WD explanation?

[John A Legelis <jlegelis@fmrco.com>]

Is there a decent explanation on the 'net on the differences between 4WD 
and AWD?  I'm asking because I was riding in my in-laws Explorer this 
weekend (the A4Q couldn't handle 5 people plus ski equipment) when the 
topic came up.  I found that I couldn't quite explain the difference 
between the Explorer's button-on-the-dash 4WD as compared with Quattro, 
4-Matic, BMW's AWD, Volvo's etc...

Please, I know this has been gone over in prior digests so I don't want to 
start another Thread-From-Hell - I'm guessing this info probably exists in 
a FAQ somewhere and am hoping someone can point me at the location.
Thanks,
John Legelis
Boston