1. Primary Domain Controler. This stores security and user account info for a domain. It manages all aspects of user/domain interaction. A domain has one PDC and can have multiple BDCs. BDCs hold backup copies of the PDC's info and are occasionally synchronized with the PDC. Both PDCs and BDCs can perform some tasks like user authentication. Often, a main site will have a PDC and a BDC (which can share the PDC's load for certain tasks) and remote sites will BDCs to handle certain tasks locally. From Linux Guide @FirstLinux