Véase Using OAuth 2.0 to Access Google APIs.
Google APIs use the OAuth 2.0 protocol for authentication and authorization. Google supports common OAuth 2.0 scenarios such as those for web server, installed, and client-side applications.
To get access keys, go to the Google APIs Console and specify your Google Developers Console. application's name and the Google APIs it will access. For simple access, Google generates an API key that uniquely identifies your application in its transactions with the Google Auth server.
For authorized access, you must also tell Google your website's protocol and domain. In return, Google generates a client ID. Your application submits this to the Google Auth server to get an OAuth 2.0 access token.
Casiano Rodriguez León 2015-01-07