Windows 2000 Active directory

[Brian Kelsay]

http://slashdot.org/article.pl?sid=00/08/23/2134237&mode=thread  
This story at Slashdot starts off talking about a Gov't users of Unix
needing to implement standard desktops for productivity apps (office apps
and PeopleSoft) and then he mentions Active Directory and goes into how his
Dept. is going to connect to other locations that will be using Active
Directory.

Can someone tell me what is so important about having Active Directory?  I
have never used Novell and NDS (although I know the term NDS), but have
never heard what it can do or if it is even important.  If it is so great to
have directory services why have Unix and NT networks gotten away without
having one for so long?  Also, is there a Directory Service in the works for
Linux?

Thanks, 
Brian