A web banner or banner ad is a form of advertising on the World Wide Web. This form of online advertising entails embedding an advertisement into a web page. It is intended to attract traffic to a website by linking them to the web site of the advertiser. The advertisement is constructed from an image (GIF, JPEG), JavaScript program or multimedia object employing technologies such as Java, Shockwave or Flash, often employing animation or sound to maximize presence. Images are usually in a high-aspect ratio shape. That is to say, either wide and short, or tall and narrow, hence the reference to banners. These images are usually placed on web pages that have interesting content, such as a newspaper article or an opinion piece.

Typical web banner, sized 468x60 pixelsThe web banner is displayed when a web page that references the banner is loaded into a web browser. This event is known as an "impression". When the viewer clicks on the banner, the viewer is directed to the website advertised in the banner. This event is known as a "click through". In many cases, banners are delivered by a central ad server.

Many banner ads work on a click-through payback system. When the advertiser scans their logfiles and detects that a web user has visited the advertiser's site from the content site by clicking on the banner ad, the advertiser sends the content provider some small amount of money (usually around five to ten US cents). This payback system is often how the content provider is able to pay for the internet access to supply the content in the first place.

Web banners function the same way as traditional advertisements are intended to function: notifying consumers of the product or service and presenting reasons why the consumer should choose the product in question, although web banners differ in that the results for advertisement campaigns may be monitored real-time and may be targeted to the viewer's interests.