HTML code (sometimes javascript, php, cgi etc) for changing the banner shown on every page view. There are plenty of banner ad rotating systems on the Internet for webmasters such "Valueclick" who pay the webmaster a set amount of money for each click on a banner (10p for example). The problem with banner rotator systems like the Valueclick one is that you have little say over who the advertisers are (could be a competing web site). For this reason lots of web sites use their own banner rotating systems and negotiate the click value with the advertisers themselves.