A cookie is a piece of information in the form of a very small text file that is placed on an internet user's hard drive. It is generated by a web page server, which is basically the computer that operates a web site. The information the cookie contains is set by the server and it can be used by that server whenever the user visits the site. A cookie can be thought of as an internet user's identification card, which tell a web site when the user has returned.
There are two types of computer cookies: temporary and permanent. Temporary cookies, also called session cookies, are stored for a short time and then removed as soon as the browser is closed. Permanent cookies, also called persistent cookies, are stored for a long time on the user's hard drive and, if deleted, will be replaced the next time the respective site is visited.
For example :the first time a user visits a site they maychoose a username which is stored in the cookie, and then provide data such as password, name, address, preferred font size, page layout, etc. - this information would all be stored on the database using the username as a key. Subsequently when the site is revisited the server will read the cookie to find the username, and then retrieve all the user's information from the database without it having to be re-entered.