What is domain controller??

• Primary domain controller (PDC) and backup domain controller (BDC) are roles that can be assigned to a server in a network of computers that use the Windows NT operating system.
• Windows NT uses the idea of a domain to manage access to a set of network resources (applications, printers, and so forth) for a group of users.
• The user need only to log in to the domain to gain access to the resources, which may be located on a number of different servers in the network.
• One server, known as the primary domain controller, manages the master user database for the domain.
• One or more other servers are designated as backup domain controllers.
  The primary domain controller periodically sends copies of the database to the backup domain controllers.
• A backup domain controller can step in as primary domain controller if the PDC server fails and can also help balance the workload if the network is busy enough.