Azure AD Connect is an hybrid service that is used to connect on-premise Microsoft Active Directory to an Azure Account (AAD)
It is responsible for the synchronization and health management of AD objects
Authentication Types
Cloud-based/ Password Hash
Uses Password Hash
For on-prem identities the hash of the hash is stored in AAD
No on-prem component is involved
Pass-through
Authentication request is sent to AAD from where it is sent to on-prem AD
The result of the authentication is then sent back to AAD
The actual authentication is performed by on-prem AD
Federated
The Federation service creates a SAML token which is passed to AAD
AAD in turn creates a access and refresh token which is used by the service