If you've subscribed to any Microsoft online service, such as Azure or Office 365, you will get the free Azure AD version along with it. It has limited features:
Directory objects
User/group management (add/update/delete), user-based provisioning, and device registration
Single sign-on (SSO)
Self-service password change for cloud users
Connect (sync engine that extends on-premises directories with Azure AD)
Security/usage reports
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