What is the deal with 4 wheel and All wheel drive?

Please excuse any error in my Straight Dope etiquite, and spelling and grammar, this is my first time posting. That being done…

What is the difference between four-wheel and all-wheel drive? Is there some sort set of hidden wheels that I don’t know about? I feel a great desire to unlock this mystery of the ages. Please assist.

menashe, welcome to the Board.

I asked the same question a while back, and you can find the responses in the thread All-wheel drive v. 4-wheel drive.