Microsoft Graph is an API that provides a single single REST API endpoint (https://graph.microsoft.com) to access various Office related products and services including users, groups, mail, messages, calendars, tasks, and notes coming from services like Outlook, OneDrive, Azure Active Directory, Planner, OneNote and others.
Microsoft SDKs can be used to build applications for iOS, Android, and Windows users that consume APIs via Microsoft Graph.
In this Channel 9 video, Microsoft Program Manager, Matt Geimer talks about new features of Microsoft Graph SDK and how to use them in your apps.