+[Acronym] for __P__rimary __D__omain __C__ontroller 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 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 - this is known as replication. 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. 

+Windows 2000 and XP server have done away with the concepts of primary and secondary domain controllers and now a server is either a Domain Controller, or not! :) Though, in a concept stolen from [ NDS] , [ADS] implements master replicas, read only replicas, etc on Domain Controllers .