A chain letter is a message that attempts to convince the recipient to make a number of copies and pass them on to a certain number of recipients. The "chain" is an exponentially growing pyramid that cannot be sustained indefinitely.
Printed Australian chain letter from 2006, with a five-cent coin taped to it as what it calls a "financial eye-catcher"
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.
News from My Lad by James Campbell, 1858–1859 (Walker Art Gallery)
Letter of Darius the Great to Gadatas, circa 500 BC.
The famous Einstein letter from Edward Teller and Leó Szilárd to US President Franklin Roosevelt suggesting an atomic bomb project. Click here for page 2.
A thank-you letter from Katharine Hepburn to Alan Light thanking him for his condolences in regards of Cary Grant's death