PDC – Primary Domain Controller
A computer server within a Windows domain, which is a network of computers that are grouped together logically and controlled by the PDC.
Within the domain there are various kinds of accounts, such as the most basic account of guest, but the PDC always has an administrator account that controls the overall domain resources.
PDCs are usually the first designated domain controller. If there are any others that exist they are typically referred to as BDCs, or Backup domain Controllers.
Within the PDC a master copy of user accounts is held and can be accessed or modified by the User Manager for Domains, which is a utility for maintaining user/group information.
The User Manager for Domains uses the domain security database found on the PDC to access the master copy. BDC computers can access the master copy but cannot modify it; it is read-only.
The account database is replicated to the BDCs regularly though and the BDCs main purpose is to provide backup to the PDC. BDCs can also be used to authenticate users attempting to log into the network.
In the event a PDC fails one of the BDCs is used to take its place.