Despite their popularity among advertisers, banner ads appear to be only mildly effective. As users have become more technically sophisticated, they have started to turn off the graphics or the autoload image option on their browsers, or they simply ignore the ads.
Portal sites such as Yahoo and America Online sell impressions, or viewings, of banner ads that are displayed on their Web sites. Companies pay for the impressions based on the cost per thousand (CPM) impressions. In addition, portals often charge for the number of clicks or referrals on an ad. In the early days of banner ads and portals ads, there was often controversy about the accuracy of measuring impressions and clicks, but third-party specialists and more mature technology have largely fixed those difficulties.

One method being used to measure the success of banner ads is called the click-through rate. A click-through of three percent is considered high in the Web advertising industry. In other words, for every 100 Web users who see the ad, only three are likely to click the ad and visit the advertiser's Web site.

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, HTML 5 often employing animation, web audio for 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.