Certified Mail is a type of Special Service mail offered by the United States Postal Service and other postal services that allows the sender proof of mailing. Some in-transit tracking and delivery confirmation information may be provided, but is not guaranteed with Certified Mail. Certified Mail is only available for Priority Mail and First Class Mail letters mailed within the United States and its territories (including APOs and FPOs). Each piece of Certified Mail is assigned a unique label number which serves as an official record of mailing of the item by the Postal Service.
Modern use 
U.S. certified mail began in 1955 after the idea was originated by Assistant U.S. Postmaster General Joseph Cooper. Today, Certified Mail is primarily used for important business mailings. It is also acceptable to send U.S. Government classified information at the Confidential level using the Certified Mail service. Certified mail is used for more than important business mailings. It is used by anyone who needs or wishes to provide a tracking number to the receiver as proof of mailing. It also allows the receiver to track their package/envelope through the online system at usps.com using the unique tracking number provided by the mailer.
Certified Mail can be done either with or without "return receipt requested", often called "RRR", which costs an additional fee. The return receipt is a green postcard-sized paper that is mailed back to the sender by the post office upon receipt by the addressee as proof of delivery. Today the USPS new Return Receipt Electronic (RRE) provides electronic proof of delivery information. When the letter reaches its final delivery destination the letter carrier captures the signature of the person that accepts the letter and the information is electronically stored. Any person can sign for the item.
Several online services attempt to modernize and streamline the process of sending certified mail. Some create SaaS private databases for RRE documents.
- USPS: Eligibility for Certified Mail
- Available USPS Destinations for Certified Mail
- The United States Postal Service: An American History accessed at http://about.usps.com/publications/pub100/welcome.htm