Google OAuth API
OAuth is an open standard for authorization. OAuth provides a method for clients to access server resources on behalf of a resource owner such as a different client or an end-user. It also provides a process for end-users to authorize third-party access to their server resources without sharing their credentials, typically a username and password pair, using user-agent redirections.
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.
OAuth 2.0 is a relatively simple protocol. To begin, you obtain OAuth 2.0 credentials from the Google Developers Console. Then your client application requests an access token from the Google Authorization Server, extracts a token from the response, and sends the token to the Google API that you want to access.