Acknowledge Letter
An acknowledgement letter, also known as an receipt or a formal written notice of recognition of a fact, a situation, an event that has taken place. This letter written as a polite response to the correspondence and association between you and the recipient. It usually mentions a person or an organization and the type of help received.
Announce Letter
An announcement letter is a letter written to let others know about a certain event or happening beforehand which concerns them . Letters of the announcement are usually informal, and state clear.