A. A PDC is a Primary Domain Controller, and a BDC is
a Backup Domain Controller. You must install a PDC before any other domain
servers. The Primary Domain Controller maintains the master copy of the
directory database and validates users. A Backup Domain Controller
contains a copy of the directory database and can validate users. If the
PDC fails then a BDC can be promoted to a PDC. Possible data loss is user
changes that have not yet been replicated from the PDC to the BDC. A PDC
can be demoted to a BDC if one of the BDC's is promoted to the PDC.