A comic book writer is someone who writes stories for comic books professionally. They write stories which are converted into pictures and illustrations by artists into a book. It's a graphic book to read and has a mix of both dialogues and drawings. A comic book writer is required to write the story very detailed and elaborate the plot extensively. He/ she then submit their works to graphic designers and artists and illustrators who convert the story into a visual story with images and drawings.
Comic book writers like any other writers, do their work over a period of time and get it edited and reworked upon. After publishing the comic book, if the story is good enough there could be a whole series of stories related to the main comic book. Comic book writers now even sell their rights to filmmakers who make TV shows and feature films out of the stories and make them popular.