A postcard is a thin piece of card or paper with a picture on one side. On the other side, there is usually a space for a message or letter. People can write a short message on the postcard and then put a stamp on it. The stamp is a special kind of sticker that pays for the postcard to be sent through the mail to its recipient. Postcards are usually sent from one person to another person who lives far away. When the postcard arrives at its destination, the person who receives it can see the picture and read the message that was written.