Azure Active Directory (Azure AD) is Microsoft's cloud-based identity and access management service. It simplifies authentication for developers by providing identity as a service. It supports industry-standard protocols such as OAuth 2.0 and OpenID Connect.
Azure AD allows users to sign in and view resources. And it has features to help secure your identities, such as Identity Protection and multi-factor authentication.
Microsoft services such as Azure and Office 365 use Azure AD to store and manage users. Whenever Office 365 needs to verify a user, for example, Azure AD performs all identity and access management.