Postcards are a unique form of mail that differs from letters or parcels in several ways. They are smaller in size, typically measuring 3.5 inches by 5 inches or 4 inches by 6 inches, and have a specific format that includes a designated space for the recipient's address and a separate area for the message. Unlike letters, postcards do not require an envelope, and postage rates for postcards are generally lower than those for letters or parcels. Postcards are often used for short, simple messages or to send greetings from a specific location and are popular as souvenirs or collectibles.