Web authentication and SSO typically imply that state is maintained on the server to indicate whether the user is logged in or not. The identity provider maintains this state and the identity consumers check this state with the identity provider. The protocol and message format differ in different implementations – SAML, OAuth, OpenID and several others.
SAML is the richest, most flexible auth protocol, but also most complex to implement. It covers the most number of use cases. The security assertions about an identity are captured in an xml format which can be exchanged between providers and consumers over the web.
OAuth is simpler and requires fewer things from the implementer. OAuth 2 has become a vehicle for enterprise use cases like SAML. SDKs for OAuth are available.
OAuth 2 and OpenID both use Json WebToken (JWT) which is a JSON format specification for interoperability.
OpenID Connect is the most open and newest of the three. It reduces reliance on checking auth state with the identity provider by embedding more information in the JWT and standardizing things like scope to increase interoperability.
A key consideration when deciding on an implementation is the scalability requirement. Ideally the system is structured to keep the least amount of state (zero) on the server. This is not true of most SSO implementations.
Like a stateless NFS server, based on leases for lock state that can be refreshed, a stateless implementation for SSO is possible (classic NFS has no open, only lookup). The tradeoff is that revocation is not as easy and reporting may need to be handled differently than with a stateful implementation. Here’s a discussion – http://stackoverflow.com/questions/26739167/jwt-json-web-token-automatic-prolongation-of-expiration