In a trusted relationship, one domain may gain access to other domains. In this case, members who log into the first domain will have access to the resources in the other domains.
It is also a good idea to have at least one backup domain controller (BDC) in a domain.
Windows 2000 (W2K) approaches the domain controller concept a little differently. Unlike Windows NT, where a PDC must be accessible to make changes to the directory, W2K relies on Active Directory’s multimaster replication model to update all domain controllers when only one is updated. There is no PDC or BDC. All domain controllers are equal.