A Web Banner or Banner Ad is a form of graphical advertising on the Internet. This form of online advertising involves embedding an advertisement into a web page. It is intended to attract traffic to a website by linking visitors to the website of the advertiser.

The advertisement is constructed from an image (GIF, JPEG, PNG), JavaScript program or multimedia object that employs technologies, such as Java, Shockwave or Flash, and often uses animation or sound to maximise its presence. Images are usually in a high-aspect ratio shape (ie. 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 newspaper articles or opinion pieces.

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 within 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 can detect that a web user has visited the advertiser's website from the original content website by clicking on the banner ad, the advertiser pays the content provider  a small fee (usually about 5 to 10 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.

Another method is one that works on a cost per impression. The pricing will depend on the website that it appears on or on the network system being used to display the ads. Other pricing models simply work on a fixed cost for the ad to appear on a website for a specified period.

Web banners function the same way that 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. They differ in that the results for the banner advertisement campaign may be monitored in real-time and that they may be targeted to the visitor's interests.

Many web surfers regard these advertisements as highly annoying because they detract from a web page's actual content and they also waste bandwidth. Of course, the purpose of the banner ad is to attract attention - without attracting attention, which would provide no revenue for the advertiser or for the content provider. Most web browsers often include options to disable pop-ups or block images from selected websites.

Web banner advertisements are restricted by high cost and limited physical banner area.

Standard Sizes

Ad sizes have been standardised, they are (in pixels):

Rectangles and Pop-Ups

• 300 x 250 - (Medium Rectangle)
• 250 x 250 - (Square Pop-Up)
• 240 x 400 - (Vertical Rectangle)
• 336 x 280 - (Large Rectangle)
• 180 x 150 - (Rectangle)

Banners and Buttons

• 468 x 60 - (Full Banner)
• 234 x 60 - (Half Banner)
• 88 x 31 - (Micro Bar)
• 120 x 90 - (Button 1)
• 120 x 60 - (Button 2)
• 120 x 240 - (Vertical Banner)
• 125 x 125 - (Square Button)
• 728 x 90 - (Leaderboard)

Skyscrapers

• 160 x 600 - (Wide Skyscraper)
• 120 x 600 - (Skyscraper)
• 300 x 600 - (Half-Page Ad)