An Acknowledgement Letter is a formal notice of recognition. It is a formal document to acknowledge the receipt of something. Usually, Acknowledgement Letters can be of different types such as sending or receiving documents, for projects, for business, for a resignation letter, etc. In general, these are short and mainly serve the legal purpose to confirm receipt of certain documents.
An announcement these days isn’t just confined to what the Royals have to say. You can announce the promotion of an employee, the hiring of a new employee, a job opening, a new policy, launch of a new product/service, retirement, upcoming event, etc. Not just for these, you can even write an announcement letter in response to a rumor, about your wedding, the death of a known person, baptism, graduation, birthday, etc. A letter announcing any of these can be an announcement letter.