Azure AD Connect is a tool and guided experience for connecting on premises identity infrastructure to Microsoft Azure AD. The wizard deploys and configures pre-requisites and components required for the connection, including sync and sign on. Azure AD Connect encompasses functionality that was previously released as Dirsync and AAD Sync. These tools are no longer being released individually, and all future improvements will be included in updates to Azure AD Connect.
Azure AD Connect GA was released to the public on 24 June 2015
The GA release offers the following high level options:
Video Azure AD Connect
Dirsync Upgrade
Organizations with an existing Dirsync deployment can upgrade in place (for directories with less than 50,000 objects) or otherwise migrate their Dirsync settings to Azure AD Connect
Maps Azure AD Connect
Express Settings
Express Settings is the default option and deploys sync with the password hash sync option for a single forest. This allows sign on to cloud resources based on Active Directory passwords.
Custom Settings
With custom settings, the administrator can connect one or multiple Active Directory forests and choose between password hash sync, pass-through authentication, and Active Directory Federation Services (AD FS) for sign on. Custom settings also allows the administrator to choose sync options such as password reset write back and Exchange hybrid deployment.
What it does
When an administrator installs and runs the Azure AD connect wizard, it performs the following steps:
1) Installs pre-requisites like the .NET Framework, Azure Active Directory Powershell Module and Microsoft Online Services Sign-In Assistant
2) Installs and configures the sync component (formerly named AAD Sync), for one or multiple Active Directory forests, and enables sync in the Azure AD tenant
3) Configures either password hash sync or AD FS with Web Application proxy, depending on which sign-on option the administrator has chosen, and including any required configuration in Azure
References
Source of the article : Wikipedia