A certified letter is sent via a shipping service offered by the United States Postal Service. This shipping option gives senders assurance that the letter will be delivered. Certified mail is often used when the contents of the letter are of higher importance than standard letters.
Certified letters can also be sent with an option for a receipt to be returned to the sender confirming delivery. In this case, the recipient has to sign for the letter and the sender is sent a card acknowledging this. This option is most commonly used when a sender requires proof of delivery for legal reasons.