Azure AD Connect is a service that you can use to synchronize your on-premises Active Directory identities with Azure. This way, you can use the same identities for authentication on your on-premises environment as well as in the cloud and other Software as a Service (SaaS) applications.

The Azure AD Connect sync service consists of two parts: the Azure AD Connect sync component, which is a tool that is installed on a separate server inside your on-premises environment, and the Azure AD Connect sync service, which is part of Azure AD. The sync component can sync data from Active Directory and SQL Servers to Azure. There is also a third component named the Active Directory Federation Services (ADFS) component, which can be used in a scenario where ADFS is involved. To monitor the on-premises identity infrastructure and the different Azure AD components, you can use a tool named Azure AD Connect Health. The following diagram illustrates the architecture of Azure AD Connect:

Azure AD Connect offers support for your users to sign in with the same passwords to both on-premises and cloud resources. It provides three different authentication methods for this: the password hash synchronization method, the pass-through authentication method, and the Federated SSO method (in conjunction with ADFS).