Google had added application sign-in to the Google+ platform to give you a way to let users sign in to your Android, iOS or Web apps with Google.
This new feature means users can sign to Google+, then use their existing credentials to sign in to apps outside Google, bringing their Google+ info for use within the other apps.
This brings Google into line with similar authentication features provided by sites such as Facebook or Twitter.
In the case of Google+, the user chooses which data to share with your apps by setting Google+ permissions, and Google+ Sign-In gives users the same protection and safeguards such as two-step verification that they’d get while working in Google+.
When a user first signs in to a website or app, they are shown a permissions screen with details of what your app would like to have permission to do. The user then has choices about what permission to grant or withhold, so they might choose to prevent your app finding out who else the user is connected to on Google+, for example.
The feature also supports single setup across browser, phone and tablet. When a user signs into your website with Google, they can install the equivalent mobile app on their Android device with a single click. Google+ Sign-In controls the activity updates of your app, so users can choose which Google+ circles will see information from the apps, and the update information will be shown only when relevant.
One potentially very attractive aspect of Google+ Sign-In is the fact that when a users shares information from your app using Google+ Sign-In, their friends will see a new kind of "interactive" post in their Google+ stream. Clicking will take them inside the app, where they can buy, listen to, or review (for instance) exactly what the original user shared.
More details of how to use about Google+ Sign-In and in your Android, iOS or Web apps can be found on Google Developers. In this video a web developer outlines the advantages of using the new facility.
As the top comment on You Tube puts it,- "about time this arrived".