Social login is a single sign-on (SSO) technology that allows users to authenticate themselves on various applications and sites by connecting through a social media site rather than typing a separate ID and password on each website.
The sites most commonly associated with social login are Facebook, LinkedIn, Google and Twitter. When the user visits a site that offers social logins, they have the option to register, log in with their regular ID and password or through a widget or plug-in that connects the site to their choice of social platform. Standards used to exchange social login information include OpenID Connect and OAuth.
The user's basic information is shared, which often means that they don't have to register for the site separately. If the user's account on the social site is configured for automatic login, they don't have to enter an ID and password on the new site at all. The ability to access social data also makes it easier for users to share information among sites and for site owners and marketers to personalize content and offers.
Social login is convenient for users but it is not considered a secure type of authentication and should never be used for any site that has sensitive information associated with it. The method also has implications for user privacy because people typically share personal information there that may not be appropriate for other sites.
Gigya provides a brief introduction to social login: