For apps requiring authentication Portals is using OpenID Connect as the default method for authenticating users. One advantage of OpenID Connect as an authentication service is that, you, our Portals customer, can leverage your preferred authentication service from a variety of providers including Microsoft, Google, Yahoo and others.
OpenID Connect differs from OAuth 2.0 in that OpenID Connect is an authentication service (authenticating users) and OAuth 2.0 is an authorization service (authorizating access to resources). OpenID delegates authentication services to your preferred provider eliminating the need to store local credentials on Portals. It also allows you to use one set of credentials across a variety of services, simplifying administration and security.
OpenID Connect sits on top of the OAuth 2.0 protocol as a simple identity layer. Software clients, such as Portals can verify the identity of your end user by leveraging an OpenID Authorization Server. It can also obtain basic profile information about your end user.
There are several good articles on the subject with a lot more detail. A few are pasted below for your convenience.