A postage stamp is a small piece of paper that is purchased and displayed on an item of mail as evidence of payment of postage. Typically, stamps are printed on special custom-made paper, show a national designation and a denomination (value) on the front, and have an adhesive gum on the back or are self-adhesive. Postage stamps are purchased from a postal administration or other authorized vendor, and are used to pay for the costs involved in moving mail, as well as other business necessities such as insurance and registration. They are sometimes a source of net profit to the issuing agency, especially when sold to collectors who will not actually use them for postage.