A banner ad or web banner 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, PNG), 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 (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.

Typical web banner, sized 468×60 pixels.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.

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.

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. Without attracting attention, which makes it annoying, 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.

Standard sizes

Ad sizes have to some extent been standardized; they are (in pixels):

Sizes for rectangular/pop-up ads

Medium Rectangle: 300 by 250
Square Pop-Up: 250 square
Vertical Rectangle: 240 by 400
Large Rectangle: 336 by 280
Rectangle: 180 by 150

Sizes for banner/button ads

Full Banner: 468 by 60
Half Banner: 234 by 60
Micro Button: 80 by 15
Micro Bar: 88 by 31
Button 1: 120 by 90
Button 2: 120 by 60
Vertical Banner: 120 by 240
Square Button: 125 square
Leaderboard: 728 by 90

Sizes for "skyscraper" ads

Wide Skyscraper: 160 by 600
Skyscraper: 120 by 600
Half Page Ad: 300 by 600