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Magic cookies," as Netscape first called them, are small files that Web publishers can save onto visitors' computers.

Sites can use cookies to control how often an ad is shown to individual users, to target advertising and to customize content.

What's the fuss all about?

Contrary to popular belief, only the site that placed the cookie on a user's hard drive can access that information, when cookies are used properly. But some say a "hole" in the design of cookies could let rogue publishers collect personal data that users shared with other sites.

Also, some privacy advocates believe that any surreptitious file that publishers slip to users without their knowledge is upsetting.

What's a "third-party" cookie?

Third-party cookies are sent to a user from a domain other than the site the user visited.

An ad network could serve a cookie to a user along with an ad when the user visits one of the sites in the ad network. The network could then reference the visitor's same cookie data when it serves him or her an ad on another site in the network. Privacy advocates find this objectionable.

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