Such domains have at least a Primary Domain Controller, and will often have one or more Backup Domain Controllers (BDCs). 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.
In later releases of Windows, such as Windows 2000, NT 4 type domains have been superseded by Active Directory. In Active Directory domains, the concept of Primary and Backup Domain Controllers doesn't exist. Instead, the domain controllers in these domains are all considered to be equal in that all controllers have full access to the accounts databases stored on their machines.
However, in these later releases of Windows, an Active Directory FSMO role named PDC emulator master does exist in each domain. This PDC emulator master does not have the same special role in replication as the Primary Domain Controller in pre-Windows 2000 systems, but does have certain additional responsibilities: