The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal systems have generally been established as a government monopoly, with a fee on the article prepaid. Proof of payment is usually in the form of an adhesive postage stamp, but a postage meter is also used for bulk mailing.

A letter is a written message conveyed from one person to another through a medium. Something epistolary means that it is a form of letter writing. The term usually excludes written material intended to be read in its original form by large numbers of people, such as newspapers and placards, although even these may include material in the form of an "open letter". The typical form of a letter for many centuries, and the archetypal concept even today, is a sheet of paper that is sent to a correspondent through a postal system. A letter can be formal or informal, depending on its audience and purpose. Besides being a means of communication and a store of information, letter writing has played a role in the reproduction of writing as an art throughout history. Letters have been sent since antiquity and are mentioned in the Iliad. Historians Herodotus and Thucydides mention and use letters in their writings.