Azure AD Connect is the service responsible for integrating on-premises AD with Azure AD. Azure AD Connect replaces the previous versions of Windows Azure Active Directory Sync (DirSync) and Azure AD Sync components. Azure AD Connect has three main components:

• Synchronization services: This service checks whether Azure AD has the same identities as on-premises AD. If it doesn't, it will create the relevant objects in Azure AD.
• Federation service: Azure AD Connect can be configured to connect via an on-premises Active Directory Federation Services (AD FS) farm. This is used by organizations that use domain join SSO, third-party MFA, smart cards, and so on.
• Monitoring: Azure AD Connect Health monitors the health of Azure AD Connect and its components. These stats can be view using the Azure portal.