A backup domain controller (BDC) is a role a Windows NT computer takes on to help manage access to network resources. The BDC maintains a read-only copy of a user accounts database and verifies logons from users. The read-only copy of the database is automatically synchronized with primary domain controllers (PDCs).
BDCs can be promoted to PDCs if the network is busy or if the PDC fails. Microsoft recommends that the promotion take place when the PDC is active so no information will be lost when it is demoted to a BDC.