Whats the use of social network login or connect [closed]

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I have seen in many websites, that they uses facebook login button, yahoo login button and twitter login button

1. what is the use of those
2. Will those sites store the authenticated user's id as username, if so what they use for password ?

thanks in advance

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There is a very good explanation here for "Login with Facebook" buttons that you see around: http://developers.facebook.com/docs/guides/web/

"Facebook helps you simplify and enhance user registration and sign-in by using Facebook as your login system. Users no longer need to fill in yet another registration form or remember another username and password to use your site. As long as the user is signed into Facebook, they are automatically signed into your site as well. Using Facebook for login provides you with all the information you need to create a social, personalized experience from the moment the user visits your site in their browser."