A scriptwriter writes the story and the spoken words for a film or a television programme. Those pages are the script: they tell the actors what to say, and they tell everyone else what happens next. In film work people often say screenwriter; in television they often just say writer. The job idea is the same. A scriptwriter is not usually the person holding the camera, and not always the person who first thought of the idea — though actually, on a small film, one person may wear several of those hats. The word is ordinary and professional, not slang.