A cookie is a small file stored on an individual's computer allowing a site to tag the browser with a unique identification. There's more to it than that though. A cookie is information your browser stores to remember preferences, where you have been on a site, or other things. The information is available only to the site that set the cookie, with the caution that there are some ways for other sites to get information detailed in the Doubleclick example further on in the discussion. According to an article written by Paul Bonner for Builder.
Cookies are, by default, stored in memory, and last only as long as the browser session.
There are ways to create a cookie that has an expiration date in the future. The cookie, with the future expiration date, is stored on a file on your hard drive. These are the cookies that I will be talking about. The Netscape 'cookies.txt' file is a text file. You can view the contents with any text editor.