Azure Active Directory meant to be a secure authentication store, which contain users and group.AAD is a cloud-based identity management store for modern applications. AAD allows you to create users, groups, and applications that work with modern authentication mechanisms like SAML and OAuth.
Azure AD Domain Services meant for If you need more than just user management, then it is possible to extend Azure AD to offer more AD based services using Azure AD Domain Services. AAD DS is an Azure product that you enable on your virtual network which deploys two domain controllers. They are managed by Microsoft and synchronized with your Azure AD tenant
