An art director is basically the person directing the project. Differs from job to job, but usually you manage a team of players, photographer, videographer, designers, etc. Each project is different too. Also, usually you work up to becoming one at an agency, steps are junior designer, designer, senior designer, manager, creative director/art director. If you were to do a freelance job and you are in charge of a lot of parts of the project, putting "art director" is a normal thing that people do. Good luck!