A cookie is a small amount of data that is sent to your web browser from a web server. This data is never "executed" as code, so it can't contain programs or viruses, and can't be longer than 4,000 characters. Cookies are generally used to help web sites determine the state of your "client."
Let's say, for instance, that you go to a shopping web site and purchase something. Instead of going off to pay for that item right away, you want to continue browsing. A cookie can be "set" to indicate that you put something in your "shopping cart", so that when you go to pay for all your items, the server has a way of knowing all the items you selected by looking at your cookie.
Yahoo! uses a cookie simply to record your Yahoo! ID. We do this so that wherever you go in My Yahoo, Yahoo Chat, Yahoo Classifieds or Yahoo! Finance, we know exactly what to display for you: your stock quotes, your sports, weather, news, sites, your personal e-mail, chat rooms, etc.
First, we should make it clear that we only use cookies to help identity you to our system. When you first register with My Yahoo, Yahoo Chat, Yahoo Classifieds or Yahoo Finance, the feedback that you provide us helps us figure out what kind of news and information we should send you. We "set a cookie" on your browser with your UserID so that when you visit any of the previously mentioned Yahoo properties, we know it's you and not your neighbor.
We also use cookies so that you don't have to bother logging in everytime when you visit Yahoo!. We should also make clear that we do _not_ distribute your cookie information to other organizations. No other sites outside yahoo.com can access this cookie.
A cookie cannot but used to extract data from your system. When a cookie is set, your browser checks it for length, an expiration date, and the domain from which it came. This domain checking means that only Yahoo can access your My Yahoo cookie. We do _not_ distribute your cookie or account information to other companies and do _not_ include your password in your cookie.
Our Yahoo! ID/password scheme is used primarily to identify you to our system. It provides some measure of security, but it is certainly not foolproof. Since we use cookies to make logging in more convenient (once you do, you don't have to again), this also means that anyone with access to your computer can see use your Yahoo account. If you want to prevent this, you can click on the "Change User" button at the lower right corner of many Yahoo! screens, to "log out" of the system and "flush" your cookie. When you return to a Yahoo, where you are required to sign in, you'll see the Sign In screen and will have to enter your Yahoo! ID and password.
We hope this information clarifies the role of cookies on Yahoo!. If you're curious, you can check out our main site directory to find other sources of information on cookies: