active directory

Active Directory is a central repository in Windows 2000. Its helps you in every single thing in win2K, whether u are going to find user, contacts, printers or you are going to create objects like user, computers, OUs etc etc.
You can enable ActiveDirectory by entering a command "DCPROMO" or by the help of configure your server wizard.

Before getting into what Active Directory is and how you configure it, I suggest that you have a look at the following article:
http://www.novell.com/coolsolutions/nds/features/tips/t_competitive_guide_edir.html.
First decide whether your organisation needs a Directory or not, then go about selecting a Directory product. For my money, there's only one choice, and it ain't AD.