There are two types of groups in Active Directory: distribution groups : You can use distribution groups to create e-mail distribution lists security groups. security groups to assign permissions to shared resources like printers ,scanners ,servers etc.
In Windows Server 2011 it is called Active Directory.
Active Directory
Windows 7 Professional can be added to an Active Directory domain. the other versions has issue and many of the feature not available to run AD correctly
the Domain is called as the security boundary with in the active directory. A Windows domain is a logical group of computers running versions of the Microsoft Windows operating system that share a central directory database. domain governs the way user access resources of the network
windows nt 4 and windows 2000
the Domain is called as the security boundary with in the active directory. A Windows domain is a logical group of computers running versions of the Microsoft Windows operating system that share a central directory database. domain governs the way user access resources of the network
The Restartable Active Directory, that allows you to have the ntds.dit in offline mode WITHOUT rebooting the domain controller.
Windows 2000 Mixed: This is the default functional level
Active Directory was previewed in 1999, released first with Windows 2000 Server edition, and revised to extend functionality and improve administration in Windows Server 2003. Additional improvements were made in Windows Server 2003 R2. Active Directory was refined further in Windows Server 2008 and Windows Server 2008 R2 and was renamed Active Directory Domain Services. Active Directory was called NTDS (NT Directory Service) in older Microsoft documents. This name can still be seen in some Active Directory binaries.
Active Directory Services is included with most Windows systems as it is a product of Microsoft. It is often used with Windows domain networks, authorizing computers within the network.
Domain Controller
Restartable Active Directory