Microsoft Graph: Unified Office 365 API

Microsoft Graph - Home
The easiest way to call Microsoft APIs
Users, Groups, Mail, Calendars, Contacts, Files,
Tasks, People, Notes and more —— all from a single endpoint.

Microsoft Graph - Documentation - overview
"Microsoft Graph (previously called Office 365 unified API) exposes multiple APIs from Microsoft cloud services through a single REST API endpoint (https://graph.microsoft.com). Using the Microsoft Graph, you can turn formerly difficult or complex queries into simple navigations."

There is a potential naming confusion, since there is Azure AD Graph API that is an API for accessing Azure Active Directory (that by the way is different from Windows Active Directory :)

Today at Connect()—introducing the Microsoft Graph - Office Blogs