Overview
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 (a tree graph) that cannot be sustained indefinitely.
Common methods used in chain letters include emotionally manipulative stories, get-rich-quick pyramid schemes, and the exploitation of superstition to threaten the recipient. Originally, chain letters were letters sent by mail; today, chain letters are often sent electronically via email, social network sites, and text messages.
Types
There are two main types of chain letter:
Hoaxes: Hoaxes attempt to trick or defraud users. A hoax could be malicious, instructing users to delete a file necessary to the operating system by claiming it is a virus. It could also be a scam that convinces users to spread the letter to other people for a specific reason, or send money or personal information. Phishing attacks could fall into this.
Urban legends: Urban legends are designed to be redistributed and usually warn users of a threat or claim to be notifying them of important or urgent information. Another common form are the emails that promise users monetary rewards for forwarding the message or suggest that they are signing something that will be submitted to a particular group. Urban legends usually have no negative effect aside from wasted time.
From Wikipedia (CC BY-SA 4.0).