As explained on the What is OpenID page:
OpenID eliminates the need for multiple usernames across different websites, simplifying your online experience.
We've added an OpenID section to the Yahoo! Developer Network that provides more details for anyone wishing to integrated with Yahoo! using OpenID. Included there is a FAQ that covers a few things you may need to know.
We're looking forward to a whole new wave of lightweight personalized applications as adoption of OpenID increases around the Web.
Yahoo! Developer Network