Postal delivery is a service provided by the postal system that involves the transportation of MAIL, parcels, and certified letters. MAIL is typically composed of letters, postcards, and other documents, while parcels are packages containing items such as merchandise or gifts. Certified letters are documents that require a signature upon delivery and are often used for legal purposes. Postal delivery is an important part of the postal system, providing a reliable way to send and receive MAIL and parcels.