Different FSMO roles.
query : netdom query FSMO (from a machine in the domain)
Schema Master – One master role holder per forest. The schema master FSMO role holder is the domain controller responsible for performing updates to the directory schema.
Domain Naming Master – One master role holder per forest. The domain naming master FSMO role holder is the DC responsible for making changes to the forest-wide domain name space of the directory.
Infrastructure Master – One master role holder per domain. The infrastructure FSMO role holder is the DC responsible for updating an object’s SID and distinguished name in a cross-domain object reference.
RID Master – One master role holder per domain. The RID master FSMO role holder is the single DC responsible for processing RID Pool requests from all DCs within a given domain.
DC Emulator – One master role holder per domain. The PDC emulator FSMO role holder is a Windows 2000 DC that advertises itself as the primary domain controller (PDC) to earlier version workstations, member servers, and domain controllers. It is also the Domain Master Browser and handles password discrepancies.
You can find netdom in the Windows XP Service Pack 2 Support Tools