Google has uploaded a nice diagram and document explaining their whole process for Account Authentication Proxy for Web-Based Applications.
They also have a document explaining the Account Authentication for Installed Applications.
The Google Account is a secured account that ties in all of Google's services protected by Google authentications.
The authentication diagram illustrates the process that the user, and Google goes through to access Account information, and the particular Google service the user is accessing. It walks through the Google Account authentication method for web based applications.
