What Is a Yahoo ID?
A Yahoo ID is a username customers need in order to access Yahoo services such as Yahoo Mail, Yahoo Answers, Yahoo Messenger and the photo service Flickr. Registering for a Yahoo ID is free, takes only a few minutes, and gives users access to a wide variety of services.
Yahoo also allows customers to use their Yahoo ID to access services not provided by Yahoo, in the same way Facebook does with Facebook Connect. This process is called OpenID, and it is meant to simplify a registration process and avoid the necessity of having several usernames and passwords across the Internet.