Azure Active Directory (Azure AD) is Microsoft’s cloud-based identity and access management service. Azure AD helps your employees sign in and access resources in:

• External resources, such as Microsoft Office 365, the Azure portal, and thousands of other SaaS applications.
• Internal resources, such as apps on your corporate network and intranet, along with any cloud apps developed by your own organization.

You can use the various Microsoft Cloud for Enterprise Architects Series posters to better understand the core identity services in Azure, Azure AD, and Office 365.