OpenID is a a single sign-on system which allows you to log in to hundreds of websites like Blogger, WordPress, Digg, AOL, Plaxo, Technorati, dzone with one username and password.
Yes only one username + password for hundreds of websites. No need to create and manage usernames + passwords for every single website separately. That's great right?
Here is a video which explains more about OpenID in very simple and plain English
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