Not a lot. I've been both. Mostly depends on the kind of business you're working for and what they prefer to call the position and what they want to include in your duties. The Creative Director, I was more responsible for coming up w/ marketing materials. Art Director I was more just over the Art department but still delt with projects .

Responsibility-wise they are fairly similar. In my experience art directors are generally more in the graphic design, television, and film fields whereas creative directors are more in the realm of advertising. There are a lot of exceptions, of course, but that is generally the trends I've noticed. Regardless, the two positions are fairly similar when it comes to responsibilities.