Google has introduced a new API for developers to take advantage of: the People API. The purpose of the People API is to simplify gathering information about a user's contacts once they have given permission. As Google points out in its announcement post, the People API essentially condenses the current process of tapping into two different APIs (Google + and Contacts) for the same information.
Previously, developers had to make multiple calls to the Google+ API for user profiles and the Contacts API for contacts. The new People API uses the newest protocols and technologies and will eventually replace the Contacts API which uses the GData protocol.
For example, if your user has contacts in her private contact list, a call to the API (if she provides consent to do so) will retrieve a list containing the contacts merged with any linked profiles.
In addition to streamlining the process of gathering contact data, Google says that the new People API exposes new data as well, such as addresses, birthdays and more. For the full details of the new API, you can check out Google's full blog post.