x.500 is a standard for directory services http://en.wikipedia.org/wiki/X.500
x.400 is a standard for messaging/communication http://en.wikipedia.org/wiki/X.400
The wikipedia entries on the subjects are good.
You can think of x.500 as the base for Active Directory. Want to read about the history of Active Directory and how it came about and how these standards were incorporated.
http://blog.joeware.net/2008/08/11/1420/
Joe summarized a thread from the activedir mail list...best discussion I've ever seen on the history of AD.
Thanks
Mike
