The People API lets you list authenticated users' Contacts and retrieve profile information for authenticated users and their contacts.
For example, let's say the authenticated user, Jen, has Fabian and Ranjith in her private contacts. When your app calls people.connections.list to retrieve a list of her connections, Jen is presented with a consent screen asking to give the app access to the list. If Jen consents, the app retrieves a list containing Fabian and Ranjith (with a resource name for each person). The app can then call people.get, passing in a resource name, to get private contact and public profile data for each person.