Google Contacts API is a RESTful web service that can be used by developers to integrate contact management into their applications. It provides a way to read and write data in users’ Google Contacts accounts on behalf of the user. For example, an application can read all of the contacts in a user’s account and offer them for export as CSV or vCard files. The application can also create new contacts on behalf of the user, update existing ones, delete unwanted ones, and even send email messages from those accounts.
OAUTH 2.0 is an authorization framework that allows users to share private data without having to share passwords or other credentials with third-party applications such as this one. It uses tokens generated by the server for every request it sends back to the client so that only authorized requests are accepted by the server and sent.