oAuth.txt

OAuth

The following steps show how your application interacts with Google's OAuth 2.0 
server to obtain a user's consent to perform an API request on the user's behalf.
 Your application must have that consent before it can execute a Google API request
  that requires user authorization.

The list below quickly summarizes these steps:

Your application identifies the permissions it needs.
Your application redirects the user to Google along with the list of requested permissions.
The user decides whether to grant the permissions to your application.
Your application finds out what the user decided.
If the user granted the requested permissions, your application 
retrieves tokens needed to make API requests on the user's behalf.