A Primary Domain Controller (PDC) is a server computer in a Windows domain. A domain is a group of computers (technically named a "forest"), where access to a variety of computer resources is controlled by the PDC. Various account types exist in the domain, the most basic is the "guest" or "anonymous login" account. The PDC has an administration account which has overall total control of the domain resources.

PDCs

Domains have at least a Primary Domain Controller, and will often have one or more Backup Domain Controller

Backup Domain Controller

In Windows NT 4 server domains., the Backup Domain Controller is a computer that has a copy of the user accounts database. Unlike the accounts database on the Primary Domain Controller , the BDC database is a read only copy...

s (BDCs). The first Windows NT Server in the domain is configured as a primary domain controller (PDC). The User Manager for Domains utility is used to maintain user and group information for the domain using the domain security database on the primary controller. The PDC has the master copy of the user accounts database which it can access and modify. The BDC computers have a copy of this database, but these copies are read-only. The PDC will replicate its account database to the BDCs on a regular basis. The BDCs exist in order to provide a backup to the PDC, and can also be used to authenticate users logging on to the network. If a PDC should fail, one of the BDCs can then be promoted to take its place. The PDC will usually be the first domain controller that was created unless it was replaced by a promoted BDC.

PDC emulation

In later releases of Windows, domains have been supplemented by the use of Active Directory

Active Directory

Active Directory is a directory service created by Microsoft for Windows domain networks. It is included in most Windows Server operating systems. Server computers on which Active Directory is running are called domain controllers....

 services. In Active Directory domains, the concept of primary and secondary domain controller relationships no longer applies. Primary domain controller emulators hold the accounts databases and administrative tools. As a result a heavy workload can slow the system down. DNS (domain name server), software may be installed on a secondary emulator machine to relieve the workload on the PDC emulator. The same rules apply; only one PDC may exist on a domain, but multiple replication servers may still be used.

• The PDC emulator master acts in place of the Primary Domain Controller if there are Windows NT
  Windows NT
  Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
  
   4.0 domain controllers (BDCs) remaining within the domain, acting as a source for them to replicate from.
• The PDC emulator master receives preferential replication of password changes within the domain. As password changes take time to replicate across all the domain controllers in an Active Directory domain, the PDC emulator master receives notification of password changes immediately, and if a logon attempt fails at another domain controller
  Domain controller
  On Windows Server Systems, a domain controller is a server that responds to security authentication requests within the Windows Server domain...
  
  , that domain controller will forward the logon request to the PDC emulator master before rejecting it.
• The PDC emulator master also serves as the machine to which all domain controllers in the domain will synchronise their clocks. It, in turn, should be configured to synchronise to an external NTP
  Network Time Protocol
  The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
  
   time source.

Samba

PDC has been faithfully recreated on the Samba emulation of Microsoft's SMB

Server Message Block

In computer networking, Server Message Block , also known as Common Internet File System operates as an application-layer network protocol mainly used to provide shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network. It also provides an...

 client/server system. Samba has the capability to emulate an NT 4.0 domain, running on a Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

machine.