From their site: You build great web applications. We have millions of users who store their data on Yahoo!. Browser-Based Authentication (BBAuth) makes it possible for your applications to use that data (with their permission). BBAuth also offers a Single Sign-On (SSO) facility so that existing Yahoo! users can use your services without having to complete yet another registration process. The first time a user visits your web site, you redirect them to a specially constructed Yahoo! URL where they can login and grant your application permission.
Yahoo took its second major step this year in opening up its service to the wider web, under the rubric of Y!OS (Yahoo Open Strategy, introduced here), with the release today of the Yahoo Address Book API.