Thus spake wikipedia

A copywriter is the person behind every word written in an advertising or publicity text (e.g., copy for press releases, posters, radio and tv announcements and so forth). Teaming up with the art director, the copywriter is one half of the creative couple. With the advent of the Internet, the role of a copywriter has become increasingly more specialized in the world of communication as the copywriter has been carving out an ever more autonomous space within the content management process.