A Primary Domain Controller (PDC) is a server that tracks changes made to accounts of all computers on a domain. It is the only computer to receive these changes directly, and therefore it serves as the account administrator for your domain. A domain has only one PDC.

A Backup Domain Controller (BDC) is a server that maintains a copy of the PDC directory database. This copy is synchronized periodically and automatically with the PDC. BDCs also authenticate user logon operations and can be promoted to function as PDCs as needed. Multiple BDCs can exist on a domain.

A Stand-Alone Server is a server that serves as neither a PDC or BDC.