Manage users and permissions with Azure Active Directory.

Windows Active Directory (AD) is not the same as Azure Active Directory.
Different skillset from AD to Azure AD.
Every Azure account will have an Azure AD service.
A Tenant is a dedicated instance of Azure AD. It represents your organization in Azure.
A user can be a member or guest of up to 500 tenants.
A Subscription is a billing entity. All resources belong to a single subscription.
Azure AD can help manage users in a hybrid cloud setup.

References