OpenID Basic Features
Most websites require a user to login before they can start using their website. So,users are forced to signup at every single website they intend to use.The signup process is usually a time consuming process since email verification is mostly necessary in order to verify the validity of the user.

Yahoo's id can be used across its network (email,groups,answers etc),and Google's id can be used on its network (gmail,gtalk,blogger etc) and Microsoft has Passport (mail,Spaces,and all MS services including Windows itself).
But what about the other majority of internet not owned by Y!,Google or MS? Its where OpenID comes to the rescue.
Started in 2005, OpenID eliminates the need for multiple usernames across different websites.Its designed by the OpenId foundation which provides it as a open source implementation. OpenID can be signed up using one of the OpenID providers. Once signed up,it can be used across the network of websites that support OpenID.OpenID foundation claims that OpenID is supported by more than 50,000 websites.
Some major websites include blogger,wordpress,livejournal,scribd,digg,dailymotion,facebook etc.,and more and more websites are starting to support OpenID gradually.
Advantages
OpenID offers users ease of use and there is no need to sign up in websites supporting OpenID.
For website owners,the advantage is that more users will be signed up for their website since users dont need to signup separately, and users only need to sign in into OpenID and join the website.
So,users can easily join an OpenId supporting website.Thats a plus for both users as well as website owners.
Moreover, OpenID can be easily implemented by website owners.
And regarding security,the OpenId password will not be submitted to the website, it will be submitted only to the OpenID provider who acts as the middleman in the sign-in process.