What is a Resignation Letters?

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.

Such a letter will often take legal effect to terminate an appointment or employment, as notice under the relevant terms of the position; many appointments and contractual employments are terminable by unilateral notice, or advance notice of a specified period of time, with or without further conditions. Even where an oral notice would be effective, the effective date or time of termination may be directly or indirectly fixed on delivery of a written letter or email, for the sake of clarity and record. In response, different arrangements may be made or agreed, such as an earlier effective date, or improved terms and conditions of appointment upon withdrawal of the letter.

It should normally be delivered in advance to the appropriate supervisor or superior, and contain such information as the intended last day at work. A period of notice may be required expressly by contract, impliedly by the pay interval, or otherwise. Nevertheless, in practice, some resignations can be effective immediately.

Sources: https://en.wikipedia.org/wiki/Letter_of_resignation