Cookies are messages that web servers pass to your web
browser when you visit Internet sites. Your browser stores
each message in a small file, called
cookie.txt. When you
request another page from the server, your browser sends the cookie
back to the server. These files typically contain information about
your visit to the web page, as well as any information you've
volunteered, such as your name and interests.
The term "cookie" is an allusion to a Unix program called Fortune Cookie that produces a different message, or fortune, each time it runs.
Cookies are most commonly used to track website activity. When you visit some sites, the server gives you a cookie that acts as your identification card. Upon each return visit to that site, your browser passes that cookie back to the server. In this way, a web server can gather information about which web pages are used the most, and which pages are gathering the most repeat hits.
Only the website that creates a cookie can read it, so other servers do not have access to your information. Additionally, web servers can use only information that you provide or choices that you make while visiting the website as content in cookies.
Webmasters have always been able to track access to their sites, but cookies make it easier to do so. In some cases, cookies come not from the site you're visiting, but from advertising companies that manage the banner ads for a set of sites (such as DoubleClick.com). These advertising companies can develop detailed profiles of the people who select ads across their customers' sites.
Accepting a cookie does not give a server access to your computer or any of your personal information (except for any information that you may have purposely given, as with online shopping). Also, it is not possible to execute code from a cookie, and not possible to use a cookie to deliver a virus.
For privacy reasons, you may wish to view the cookies currently stored in your browser or control which sites you accept cookies from. You may also decide how long they may be stored and used. Most modern browsers offer the ability to control cookie settings; consult your browser's help files, and see: