Cookies

HTTP cookies provide a web server with a mechanism to store and retrieve state data on the client application's system. This mechanism enables Web-based applications to store data about selected items, user preferences, registration data, and other data that can be retrieved later.An Internet (or "magic") cookie is a text file placed on your computer by web sites that you visit. Because it only contains text it cannot transmit a virus or do damage to your system. It may or may not be a privacy concern depending on how it is used.

Browser cookies have another delicious quality: They let Web pages communicate with each other. That wasn't possible in the Internet's early days. Nor was it necessary. The World Wide Web was originally used to publish scientific papers and other documents. When somebody clicked on a hyperlink or typed a new URL, the exact location of the information was sent to the server, and Web servers responded by sending the requested page. Web servers knew nothing about the person requesting the information. As a result they had no way to customize their replies to suit the needs of individual users.

Affairs of state

The Web's original design made online sales, interactive customer support, games and many other attractive applications impossible. To do these things, Web pages must talk to each other. For example, a page that lets you select an item to purchase must be able to pass that information to another page that displays total charges. This sort of data is what programmers call state information. It allows Web pages to be constantly apprised of the state of your visit-what decisions you've already made and what information you've already entered-and to act accordingly. 

Cookies provide a solution to the state information problem, by allowing Web sites to store small amounts of state information on your hard disk. Later, that same site, or a closely related site, can retrieve that information. Together with advances in server-side programming, cookies make many of the Web's most exciting uses possible. 

Security

Unfortunately, cookies' ability to store this information led many to believe they posed a serious security risk. Some claimed cookies allowed Web sites to learn private information about their visitors. Others suggested cookies could damage your computer by transporting and concealing a computer virus.

Fortunately, these early fears have proven unfounded. Cookies are just text files; they can't contain a virus. Cookies do contain information you've voluntarily provided a Web site by entering data in forms, visiting pages and so on. But because Web sites can only read cookies they themselves created, a cookie can't reveal anything to any other Web site.

So, far from being a threat, cookies can actually enhance security and privacy by letting Web sites store information about you on your computer's hard disk. If cookies didn't exist, this same information would be stored in a large database on the Web site's computer, far away from your control or inspection.

Some common uses for Internet cookies are:

• An anonymous code given to you so the web site operator can see how many users return at a later time. These cookies are configured to stay on your system for months or years and are called "persistent" cookies.
• A code identifying you. This usually occurs after a registration. The site could keep a detailed account of pages visited, items purchased, etc. and even combine the information with information from other sources once they know who you are.
• A list of items you purchased. This is often used in "shopping cart" web sites to keep track of your order. Often cookies of this type 'expire' as soon as you log out or after a short time. These are called "session" cookies.
• Personal preferences. This can be anonymous or linked to personal information provided during a registration.