Azure Active Directory
Azure Active Directory (Azure AD) is the identity management solution for Microsoft and non-Microsoft cloud solutions. It simplifies authentication for business-to-business (B2B) collaboration and can also be used as identity provider for on-prem systems.
Most of the organizations are using Azure AD whether they know it or not. For example, your Azure AD instance is provisioned the moment you start using Microsoft Office 365.
Azure AD has multiple editions, free edition is included in Microsoft Office 365. Paid licenses (Premium P1 or P2) include additional features like Self-service Password Reset.