Azure AD Connect, commonly known a simply: Connect Sync, is a Microsoft service that addresses operations related to synchronize identity data between your on-premises environment and Azure AD. Released late 2014, Azure AD Connect sync is the successor of DirSync, Azure AD Sync, and Forefront Identity Manager.
By default, Azure AD Connect syncs all objects if we do not customize sync options. Azure AD Connect offers such flexibility to fine tune sync settings as per business requirement.
Update: 2022/09/01: On August 31, 2022, all 1.x versions of Azure Active Directory (Azure AD) Connect will be retired because they include SQL Server 2012 components that will no longer be supported. Please upgrade to the latest release of Azure AD Connect version 2.x or Azure AD Connect cloud sync.
Azure AD Connect : Customize Sync Options – TechNet Articles – United States (English) – TechNet Wiki (microsoft.com)
AAD connect Sync Rule | Azure Active Directory Users – YouTube