What is OpenID Connect?

OpenID Connect is an authentication system that is built on top of OAuth 2.

OpenID Connect provides a way for websites and applications, who support OAuth 2, to define a standard means of using OAuth 2 to provide authentication and identification. Typically, OIDC is used to allow users to authenticate to another website or application using their existing account on a service such as Google or Facebook; since Google and Facebook "speak" OIDC, another website that wants to add a "Login with Google" button can do so without having to implement a custom authentication API.

Open ID Connect, like all authentication systems, does not generally provide authorization or permissions; while OAuth 2 does provide permissions, OIDC as a specification explicitly is designed not to do so, in favor of simply providing the identity of the user to the website or application using it.