A letter of resignation is written to announce the author's intent to leave a position currently held, such as an office, employment or commission.
A resignation letter is a formal document where an employee announces to their employer that they will be leaving their job; usually preceded by an informal face-to-face talk or handing in a resignation letter. It is seen as an act of courtesy to notify the employer that the working relationship has reached an end, whether that is for personal or professional reasons.