from A List Apart, an article that explains what art directors actually do.
What is art direction?
That’s a hard question to answer. In the movies, art directors are usually responsible for creating the “look and feel” of the film. In advertising and print work, art directors (often teamed up with a copywriter) come up with “concepts,” the creative ideas which communicate with us on a gut level through such devices as theme, metaphor, and symbolism. Some art directors do little more than dream up these ideas and present them to clients, while some oversee almost all aspects of the design and production process. Surprisingly, art direction is seldom taught in schools and there is very little formal information on the subject; it is often learned in practice.
i know i never got any formal training in being an art director -- it was completely learned through the sink-or-swim and observation methods. it's all about the concept. the art director is the person with the idea that the design hangs on; it's the conceptual solution to the problem your design needs to solve.
without that concept, without the solution -- all you have is pretty trash.