Start here -->
3.0 Third Party Cookies - DoubleClick
The following description of DoubleClick contains information from the w3.org web site.
The DoubleClick Network is a system created by the DoubleClick Corporation to create profiles of individuals using the World Wide Web and to present them with advertising banners customized to their interests. DoubleClick's primary customers are Web sites looking to advertise their services. Each member of the DoubleClick Network becomes a host for the advertising of other members of the network. When a Web site joins DoubleClick it creates advertisements for its services and submits them to DoubleClick's server. The Web site then modifies its HTML pages to include an <IMG> graphic that points to DoubleClick. When a user goes to view one of these modified HTML pages, her browser makes a call to DoubleClick's server to retrieve the graphic. The server chooses one of its member's advertisements and returns it to the browser. If the user reloads the page, a different advertisement appears. If the user clicks on the graphic, her browser jumps to the advertised site. Currently many hundreds of sites belong to DoubleClick.
From the user's point of view DoubleClick's graphics appear no different from any other Web advertisement, and there's no visible indication of anything special about the graphic. However, there is an important difference. When a user first connects to the DoubleClick server to retrieve a graphic, the server assigns the browser a cookie that contains a unique identification number. From that time forward whenever the user connects to any Web site that subscribes to the DoubleClick Network, her browser returns the identification number to DoubleClick's server, allowing the server to recognize her. Over a period of time DoubleClick compiles a list of which member sites the user has visited and revisited, using this information to create a profile of the user's tastes and interests. With this profile in hand the DoubleClick server can select advertising that is likely to be of interest to the user. It can also use this information to compile valuable feedback for its member Web sites, such as providing them with audience profiles and rating the effectiveness of the advertisements.
ad.doubleclick.net FALSE / FALSE 942195440 IAA d2bbd5
then you are carrying a DoubleClick cookie.
Windows users will find the equivalent information in the file cookies.txt, located in their C:\Programs\Netscape\Navigator directory, while Macintosh users should look in their System Folder under Preferences:Netscape. Users of Microsoft Internet Explorer should examine the files located in C:\Windows\Cookies.
Current versions of both Netscape Navigator and Internet Explorer offer the option of alerting you whenever a server attempts to give your browser a cookie. If you turn this alert on, you will have the option of refusing cookies. You should also manually delete any cookies that you have already collected. The easiest way to do this is to remove the cookies file entirely.