Cookies are pieces of data placed on a computer's hard drive by a web server; they can be used for a variety of purposes. They can store usernames and passwords, for example, so that people don't have to continually log on to a site that requires registration; or they can enable people to fill electronic shopping carts with goods they want to buy. Cookies also store the name of the site that placed the cookie. Only that site can read the cookie information, so information from one site can't be shared with information from another site. Cookie information is put into a special file on a hard disk. The location and files vary according to the type of computer and the browser. On PCs using Netscape, for example, the information is put into a file called COOKIES.TXT. That single text file holds all the cookies, and each cookie is one line of data in the file.
