Certified mail is a service offered by the United States Postal Service (USPS) that provides proof of delivery and a return receipt for sent items. Certified mail is often used for important documents, such as legal paperwork or financial statements. The sender can track the delivery of their document and receive a confirmation when it is received by the intended recipient. Certified mail is more expensive than regular mail, but it offers peace of mind for senders who need to know that their document has been received.