A web banner or banner ad is a form of advertising on the World Wide Web delivered by an ad server. 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, 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.

When the advertiser scans their log files 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).[citation needed] 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, a fact first documented on Hot Wired in 1996 by researchers Rex Briggs and Nigel Hollis. Web banners differ in that the results for advertisement campaigns may be monitored real-time and may be targeted to the viewer's interests.