So although you may have never been to doubleclick.net, you probably have a cookie from them. This use of cookies is the most controversial. Because these companies put ads on hundreds of sites, they will recognize you because of the cookie and can then track your surfing habits across these sites. With the help of its online partners, DoubleClick began correlating the Abacus database of names with people's Internet activities. DoubleClick defends the practice and insists that it allows better targeting of online ads.
Consumer advocates call it an invasion of privacy.
If you are concerned about advertising companies tracking your surfing habits and want to have control over your cookies, check your browser's documentation or help files. However, it may offer only limited control. You can choose to delete cookies, but they will simply return the next time you visit the site. You also can turn them off completely, but then you may lose the functionality of most sites. Setting the preferences to ask your permission for every cookie can make Web browsing a consistently annoying experience.