Cookies typically hold such nuggets as login or registration data, what parts of a site you looked at, even what purchases you made there. Whenever you return to the site, their server looks for the cookie they left on your hard drive. Then it knows you've been there before, and has some information it can use to serve you better.
Are cookies a threat to your privacy? They could be seen that way, but the threat is far less onerous (at least for now) than the one you face when you use your credit card in off-line lifeâ€"since that data, too, can be used to compile a market profile of you and is often shared with others without your knowledge.
In most cases, though, cookies are a convenience feature. For example, after you register once with the online edition of The New York Times, you won't have to re-type all your identifying infoâ€"including the easy-to-forget passwordâ€"the next time you go to their site. The cookie will tell the Times who you are and let you in.
Also, Web browsers keep the cookies from one site hidden from all other sites' gazeâ€"that means The Wall Street Journal will never know you cheat on it by reading The New York Times.
If you're the paranoid type that doesn't want any hidden files on your computer, there are ways to block or refuse cookies. Remember, though, some sites rely on them to enhance your experience or perform various functions. Blocking cookies can make your visit there slower and clumsier. Microsoft Internet Explorer and Netscape Navigator can be set to alert you when a Web site wants to toss its cookies into your system.
If you're looking for something stronger, there are several "anti-cookie" programs you can buy. If you're the type that likes to take matters into your own hands, you can always toss your cookies yourself. On a PC, you'll find all those little files in your C:\Windows\Cookies folder.
If you're on a Mac, cookies are contained within a single file called MagicCookie. You'll find it in the following folder: System Folder/Preferences/Netscape.
