tell the server what pages to show the user so the user doesn't have to remember or start navigating the site all over again. Cookies act as a sort of "bookmark" within the site. Similarly, cookies can store ordering information needed to make shopping carts work instead of forcing the user to remember all the items the user put in the shopping cart. Persistent or tracking Cookies are also employed to store user preferences. Many websites allow the user to customize how information is presented through site layouts or themes. These changes make the site easier to navigate and/or lets user leave a part of the user's "personality" at the site. For Information on session and persistent and tracking cookies, see here Cookie security and privacy issues Cookies are NOT viruses. Cookies use a plain text format. They are not compiled pieces of code so they cannot be executed nor are they self-executing. Accordingly, they cannot make copies of themselves and spread to other networks to execute and replicate again. Since they cannot perform these functions, they fall outside the standard virus definition. Cookies CAN be used for malicious purposes though. Since they store information about a user's browsing preferences and history, both on a specific site and browsing among several sites, cookies can be used to act as a form of spyware. Many anti-spyware products are well aware of this problem and routinely flag cookies as candidates for deletion after standard virus and/or spyware scans.See here for some privacy