11.2 How do advertising cookies work?
Many websites host advertising banners, and these advertisements may place a 'third-party cookie' on your computer. This means that the cookie does not come from the website you are viewing but from one of the advertisers. There has been pressure on the browser manufacturers to control third-party cookies.
The website carrying the advertising banners may also set a cookie on your computer. The cookie records which advertisements you see as you view the web page and which advertisements you click on.
Most websites do not keep their advertisements on the website you are visiting. Instead they subscribe to a service that supplies the advertisements for them. This is accomplished via a simple HTML call to the service. When a page is requested, it is assembled through many HTTP requests by the browser. First, there is a request for the HTML itself. Then, everything the HTML needs is requested, including images, sounds, and plug-ins.
The call to the advertisement service is an HTTP request for an image. Once the request is made to the service, it can return more than just an advertisement. It can also return a cookie or, if you have seen this advert before, it can read that first, and check to see what advertisement to send. This enables targeted advertising.
All this information can be used to build up a detailed profile of your likes and dislikes, and the websites you have visited.