A shared identity service that enables users of multiple OpenID-enabled Web sites to sign in under one single profile, or single user identity. It's a free and open standard under which users are able to control the amount of personal information they provide on Web sites, and in particular social networking sites (SNS). To use OpenID across multiple sites, users must first choose an Open ID provider. Once you create an OpenID it stays with you, even if you choose at a later time to switch to a different OpenID provider.
OpenID works by using existing Internet technologies such as URI, HTTP, SSL and Diffie-Hellman to transform the information you provide on one site (or service) into an account that can be used at other sites that support OpenID logins. Some of the online and Internet services supporting OpenID include AOL, Blogger, Flickr, LiveDoor, LiveJopiurnal, Vox, Yahoo, WordPress, and others.