The phrase web banner is often used to refer to a form of advertising on the World Wide Web delivered by an ad server. It is also known as a banner ad. It is not to be confused with a non-advertising banner at the top of a web page that graphically identifies the website it is on or this website’s prime content, which type of banner is technically known as the hero image.
As a form of online advertising, the web banner, or banner ad, 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. 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.
Standard sizes: 728x90px, 468x60px, 234x60px, 120x90px, 120x60px, 120x240px i 125x125px.
Read more: http://en.wikipedia.org/wiki/Web_banner