[Simplest] Consumer reports synopsys of all-wheel drive (AWD) vs. 4-wheel drive (4WD). (You don't have to be a CR subscriber to access the page.)
[Slightly longer] Edmunds article that explains AWD and 4WD but also the major pros and cons of front- and rear-wheel drives (FWD vs. RWD).
[Longest but most detailed] Independent article explaining technical and historical aspects of 4-wheel drive systems. Talks about current Subaru, Audi, and other specific drive systems. Also, a bit more than half way through it explains why full time AWD systems or part-time automatically engaging all-wheel drive systems usually get better gas mileage than part-time manually-engaging 4WD systems like those on older SUVs. I think this article is very informative, but quite long.
Other health and safety related consumer info that I've collected.