Azure Active Directory Connect is a simple, fast and lightweight tool to connect Active Directory and other on-premises directories with Azure Active Directory in a few clicks. Azure AD Connect will guide you to synchronize only the data you really need from single or multi-forest environments and enable single sign on via password sync or federation with AD FS to Office 365 and thousands of other SaaS applications. More capabilities are constantly added like the ability to monitor your identity bridge with Azure AD Connect Health, group and device writeback and more. Organizations already using DirSync or Azure AD sync can take advantage of the benefits of Azure AD Connect with migration or in place upgrade from supported configurations.
In this video Andreas Kjellman, Sr. Program Manager from the Identity product group will demonstrate how to achieve the in place upgrade from DirSync, the legacy sync tool, to Azure AD Connect.
You can read more in the Azure AD Connect documentation pages