OpenID allows you to use an existing identity to sign in to multiple websites, without needing to create new passwords.
You may choose to associate information with your OpenID that can be shared with the websites you visit, such as a name or email address. With OpenID, you control how much of that information is shared with the websites you visit.
With OpenID, your password is only given to your identity provider, and that provider then confirms your identity to the websites you visit. Other than your provider, no website ever sees your password, so you don’t need to worry about an unscrupulous or insecure website compromising your identity.
OpenID is rapidly gaining adoption on the web, with over one billion OpenID enabled user accounts and over 50,000 websites accepting OpenID for logins. Several large organizations either issue or accept OpenIDs, including Google, Facebook, Yahoo!, Microsoft, AOL, MySpace, Sears, Universal Music Group, France Telecom, Novell, Sun, Telecom Italia, and many more.
Source: What is OpenID
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