In Windows NT, they had the concept of a Primary Domain Controller and a Backup Domain Controller. In NT on the PDC was writable and the BDC's where just a copy of the information on the PDC. This was so the the BDC could take some of the authentication load and also had a backup of the Domain User/Groups, etc... should anything happen to the PDC.
In Windows 2000+ this concept was changed so that all domain controllers where writable and replicated changes amongst each other. (There is a term for this which escapes me)