One of the challenges of working within a large, distributed computing environment is identifying and locating resources such as users, groups, print queues, and documents. A directory service is part of a distributed computing environment that provides a way to locate and identify the users and resources available in the system. The directory service is like a phone directory. Given a name for a person or a resource, it provides the information necessary to access that person or resource. You do not have to use specific binding information to access a resource on the network.
Most enterprises already have many different directories in place. For example, network operating systems, electronic mail systems, and "groupware" products all have their own directories. Many issues arise when a single enterprise deploys multiple directories. These issues include usability, data consistency, development cost, and support cost, among others.
Active Directory addresses these issues by providing a single, consistent, open set of interfaces for managing and using multiple directories.