You may have noticed that Web sites are getting smarter by the day. They seem to "know" more and more about you each time you visit. For instance, you may bookmark a popular site such as the Amazon or CDnow, and find that the computer on the other end knows not only that you've been there before, but exactly when you last visited, and what you were looking at the last time you clicked by.
Spooky, you say? Exciting? Perhaps a little of both?
Most Web sites accomplish this stunning feat with HTTP cookies. A cookie is a small piece of information that's sent to your browser - along with an HTML page - when you access a particular site. When a cookie arrives, your browser generally saves this information to your hard drive; when you return to that site, some of the stored information will be sent back to the Web server, along with your new request.
Sites with "shopping carts" are a good example of cookies in action: You browse a series of Web pages for items to buy, and when you find something you want, you "add it" to your shopping cart by clicking a button on the page. Later, you can view these items all together.
The funny thing is, even though you're communicating through an "anonymous" connection, the site always knows exactly what's in your personal shopping cart. It doesn't seem to matter whether you've clicked away to somewhere else and come back, or even if you've completely shut down your computer and returned days later. The site still knows who you are, and what you were shopping for. But how?