A banner ad is an electronic billboard or ad in the form of a graphic image; these come in many sizes and reside on a Web site's Web page.  Banner ad space is sold to advertisers to earn revenuefor the Web site.

These are typically located near the top or bottom of websites where they are easily viewed.  It is important to make your banner ad as appealing as possible to ensure that it generates traffic.  A good article on banner advertisng can be found by clicking here