A Microsoft Active Directory, in simple terms, is like a giant telephone book that organizes within it all of the computers, people, printers and other IT objects that have been entered into it. In our case our Active Directory is called Nexus. Unlike a telephone book however Nexus is not sorted alphabetically, but rather like the yellow pages by category, in our case by faculty and department. This allows us to mimic the universities administrative structure for Academic Support departments.
Administrators use an active directory to apply policies to objects (computers and users), put people into security groups (to allow and deny access to resources), and to better keep track of things in groups (called Organizational Units). Clients can make use of an active directory to look up names, phone numbers and any number of other attributes allowed by administrators.
Nexus is more than this though. It is the central authentication domain used by Quest and to log onto UW computers, to name two examples. It is synchronized regularly with WatIAM. Many corporate resources authenticate against Nexus and it makes our lives easier by giving us one central place to maintain these accounts.