Formal Letters - are written to express an opinion or to lodge an official complaint if you have been the victim of bad customer service or have received faulty products.
Personal Letters - are used to convey feelings and emotions, express grievance, to give thanks or any other matter of a personal nature.
Copywriting - is the art of using words to promote a person, business, opinion, product or idea. Well put together copywriting will by it's very nature persuade the reader.