Active Directory is called a directory service, it stores objects like users and computers.

A domain controller is the server running the Active Directory is called.

You can have multiple domain controllers for many reasons, like redundancy so should one server fail, people can still login in and access things like joined computers using another domain controller while the first server is being fixed.

Just is small example of how Active Directory is to be used:

Forest (abc.com)

Domain (newyork.abc.com)

Domain Controller D

Domain Controller C

Computer A, users can login to the computer using their account on using either Domain Controller D or Domain Controller C depending on different factors like location etc.

Computer B, same

Computer C, same

Domain controllers have rights to authenticate and authorize login of users in domain

Only one Domain Controller in a domain can handle can tasks like the Global Catalog.

Primary Domain Controller (PDC)

Infrastructure MAster

RID

Domain Master

all are domain controllers with specfic FSMO roles

A Primary Domain Controller (PDC) is a server computer in a pre-windows2000 NT server Domain. A domain is a concept used in NT server Operating_systemwhereby a user may be granted access to a number of computer resources with the use of a single username and password combination

The PDC has the master copy of the user accounts database which it can access and modify. The BDC computers have a copy of this database, but these copies are read-only. The PDC will replicate its account database to the BDCs on a regular basis. The BDCs exist in order to provide a backup to the PDC, and can also be used to authenticate users logging on to the network. If a PDC should fail, one of the BDCs can then be promoted to take its place. The PDC will usually be the first domain controller that was created unless it was replaced by a promoted BDC.

PDC emulator master does not have the same special role in replication as the Primary Domain Controller in pre-Windows 2000 systems, but does have certain additional responsibilities:

The PDC emulator master acts in place of the Primary Domain Controller if there are Windows NT 4.0 domain controllers (BDCs) remaining within the domain, acting as a source for them to replicate from.

The PDC emulator master receives preferential replication of password changes within the domain. As password changes take time to replicate across all the domain controllers in an Active Directory domain, the PDC emulator master receives notification of password changes immediately, and if a logon attempt fails at another domain controller, that domain controller will forward the logon request to the PDC emulator master before rejecting it.

The PDC emulator master also serves as the machine to which all domain controllers in the domain will synchronise their clocks. It, in turn, should be configured to synchronise to an external NTP time source.

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