Domain name Controller (Active Directory)

Domain name controller:

On Microsoft Servers, a domain controller (DC) is a server that responds to security authentication requests (logging in, checking permissions, etc.) within the Windows Server domain.[1][not in citation given][2] A domain is a concept introduced in Windows NT whereby a user may be granted access to a number of computer resources with the use of a single username and password combination.  (source: Wikipedia)

In my company: We have 5 domain controllers (ADs) - one at each site (3 sites) and 2 virtual servers acting as domain controllers (or AD - Active directory) at our hosting partner site.

All clients (like our laptops) need to be logged in with userid/pwd registered at Domain Controller (AD - Active directory).  Printers are connected to AD as well.  A client can will be connected to any one of ADs (DNC) randomly.. In case of Internet failure,  local DNC (AD) is used to access Printers etc..  Userid and Pwds are generally catched at client level (Laptop level), so you can login to system from home(with out internet) as well.. but this is only valid for few days... You cant change pwd with out connecting to domain controller.