What are ‘cookies’?

Cookies are text files that a Web server can store on a user's hard disk. Cookies allow a Web site to store information (sites visited or credentials for accessing the site) on a user's machine and later retrieve it. The pieces of information are stored as name-value pairs. Cookies are designed to be readable only by the Web site that created them. A name-value pair is simply a named piece of data. It is not a program, and it cannot "do" anything. A Web site can retrieve only the information that it has placed on your machine. It cannot retrieve information from other cookie files or any other information from your machine.