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 to the website of the advertiser. The advertisement is constructed from an image (GIF, JPEG, PNG), JavaScript program or multimedia object employing technologies such as Java, Shockwave or Flash, often employing animation, sound, or video to maximize presence. Images are usually in a high-aspect ratio shape (i.e. 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. Affiliates earn money usually on a CPC (cost per click) basis. For every unique user click on the ad, the affiliate earns money.

The 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.

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. Usually though, advertisers use ad networks to serve their advertisements, resulting in a revshare system and higher quality ad placement.

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. Behavior is often tracked through the use of a click tag.

Many web surfers regard these advertisements as highly annoying because they distract from a web page's actual content or waste bandwidth. (Of course, the purpose of the banner ad is to attract attention and many advertisers try to get attention to the advert by making them annoying. Without attracting attention it would provide no revenue for the advertiser or for the content provider.) Newer web browsers often include options to disable pop-ups or block images from selected websites. Another way of avoiding banners is to use a proxy server that blocks them, such as Privoxy.