Windows Azure Active Directory (Windows Azure AD), currently in Preview release, is a cloud-based, RESTful identity provider service that provides identity management and access control capabilities for your cloud applications. It enables you to easily integrate with on-premises AD deployments and provides full support of third party identity providers. You can manage users across their online services such as Windows Azure, Microsoft Office 365, Dynamics CRM Online, Windows Intune and other 3rd party cloud services.
You can sign up for the Windows Azure service by creating a new Windows Azure AD tenant. The signup process is very straightforward.
Once you complete the sign up, you can log in and perform common tasks such as adding users and resetting user passwords. If you have your own domain name already, you can add your own domain to improve the experience if your organization uses Exchange Online, Lync Online, or SharePoint Online. You can integrate Windows Azure Active Directory with your on-premise Active Directory with one of the two options: directory synchronization and single sign on. More detail on directory integrations here.
Developers can create applications to integrate with Windows Azure Active Directory. The web application sample available from MSDN shows how you can list and create users and groups.