What is the difference between Google Identity Toolkit, Google OAuth, Firebase Auth and Google+ sign in

For my understanding, Google Identity Toolkit provides a layer for login with different accounts, like Facebook, Yahoo etc. but how does that differ from the Google OAuth implementation? And regarding Google+ sign in, it is still using OAuth, but only enables login for Google accounts?

Update as of December 28, 2018: Google is pushing a new service called Cloud Identity Platform. Which is basically Firebase Authentication just for all GCP customers.

This service offers, like Firebase Authentication, an abstraction layer to any authentication platform (with easy 2-factor authentication starting at GA)

Google Sign-In and Smart Lock are both only the underlying "low level" technologies used by the Firebase Authentication and Cloud Identity Platform client SDKs.

Update as of June 15, 2016: It seems the guide mentioned in the original answer (below) is not kept up to date - the Google Identity Toolkit seems to be on its way out, apparently absorbed into the Firebase Authentication, which is not even mentioned in that guide.

Maybe a better starting point for checking the current Google authentication solutions might be the "Sign In + Identity" icon on the Products page.

Original answer:

I recently saw this Google Identity Platform guide for choosing between various Google identity solutions which may answer some/all of your questions: https://developers.google.com/identity/choose-auth