Banners are a small, usually longer than wider, image that appears in one's signature on a website. A banner on BZP is typically use to advertise something, such as a comedy, an RPG, or a trading topic. However, banners are also used to send out a non-personal message, express one's beliefs on something, or simply show public approval/disapproval of a person, group, or item.
It is commonly believed that banners that look better will attract more attention, and therefore more people, to the topic being advertised. Thus, banner makers are always trying to find a new style that looks good and use it. The most creative banner makers on BZP have their own "banner shops," where banners can be requested and sometimes made.