I used to manage a creative team for a textbook publisher. One day one of my designers brought her daughter, who was in third or fourth grade, into the office and introduced her to me.
"This is my boss," she told the little girl. "He's an art director. Do you know what an art director does?"
Without hesitation, the child said, "He keeps everyone from pitching a fit!"
I've never heard a better job description.

As a CD who began as an AD, my experience has been that the lower on the totem pole, the more tactical, the higher, more strategic. As a CD, I guide the creative product to ensure it is appropriate in strategy and voice. I also teach more. As an AD, I am more hands-on with concepting, hiring the right photographer, illustrator or writer, and deal more with vendors like printers, jingle writers and producers.

My opinion is that in an ad agency setting, by the time you've been in the business 3-5 years, an AD should be able to come up with headlines and copy direction (though not necessarily write finished copy), and a writer should be able to visualize and describe accurately the look and feel of an ad (though not necessarily be able to draw, design or art direct). The most effective people I've seen in either discipline have been able to do that.