Cookies are a very useful tool in maintaining variables on the Web. Cookies can store database information, custom page settings, or just about anything that would make a site individual and customizable. For example, an airline site may set a cookie indicating a particular passenger prefers aisle seating.
Cookies are small data structures delivered by a Web site to a user.
The Web site may deliver one or more cookies to the client. The client stores cookie data in one or more files on its local hard drive. A cookie is a simple piece of text. It cannot be used as a virus, and it cannot access your hard drive. There has been a concern whether cookies can read information and gather sensitive information from a user's hard drive.