NT in a Domain Environment
As a Primary Domain Controller (PDC), NT Server centrally manages all user accounts, security, and access control information.
If a second server is added, it becomes the Backup Domain Controller, with security and user account information replicated automatically from the PDC.
The BDC provides login and management functions if the PDC fails.
NT Server can also be a standalone server (i.e. a database server, a web server, or a proxy server)