Windows Server 2008 originally introduced new capabilities to start or stop directory services running on a domain controller without having to shut it down. This allows administrators to perform maintenance or recovery on the Active Directory database without having to reboot into Directory Services Restore Mode.
In addition to allowing for maintenance and recovery, turning off the domain controller functionality on an AD DC essentially turns that domain controller into a member server, allowing for a server to be quickly brought out of DC mode if necessary. Microsoft has also removed the need for local Administrators on the DC to have Domain Admin rights as well, which improves overall security in places where administration of the DC server is required, but full Domain Admin rights are not needed.
To take a Windows Server 2008 R2 DC offline, perform the following steps:
1. Open up the Services MMC (Start, All Programs, Administrative Tools, Services).
2. From the Services MMC, select the Active Directory Domain Services service. Right-click it and choose Stop.
3. When prompted that stopping AD DS will stop other associated services such as DNS, DFS, Kerberos, and Intersite Messaging, choose Yes to continue.
4. To restart AD DS, right-click the AD DS service and choose Start. Start the Intersite Messaging Service and Kerberos Key Distribution Center service as well.
Source of Information : Sams - Windows Server 2008 R2 Unleashed