Azure Active Directory (AAD) is the identity management and access control solution from Microsoft. With the ability to synchronize on-premise AD accounts to AAD, enabling multi-factor authentication, to enabling business to business & business to customer, this is going to be adopted widely. Lots of Microsoft online services (Office 365, Dynamic CRM) uses AAD under the hood.

When we think of signing up for Azure, Office 365 or Azure AD, there are lots of possible scenarios:

• I have Office 365 & I don’t have a Microsoft Azure subscription
  
• I have a Microsoft Azure subscription and I want to get Office 365
  
• I have Office 365 and I have an existing Azure subscription (not using AAD)
  

If you would like to understand the features and your options as administrator, in these scenarios, I strongly recommend you to check-out this Microsoft Virtual Academy session from Bill Fiddes. https://www.microsoftvirtualacademy.com/en-US/training-courses/support-corner-accessing-azure-ad-portal-from-office-365-10634

Also, here is the Azure documentation for various scenarios, and step-by-step guidance. https://azure.microsoft.com/en-us/documentation/articles/active-directory-howto-tenant/