A directory is a stored collection of information about objects that are related to one another in some way. For example, an e-mail address book stores names of users or entities and their corresponding e-mail addresses. The e-mail address book listing might also contain a street address or other information about the user or entity.

In a distributed computing system or a public computer network such as the Internet, there are many objects stored in a directory, such as file servers, printers, fax servers, applications, databases, and users. Users must be able to locate and use these objects. Administrators must be able to manage how these objects are used. A directory service stores all the information needed to use and manage these objects in a centralized location, simplifying the process of locating and managing these resources. A directory service differs from a directory in that it is both the source of the information and the mechanism that makes the information available to the users.

A directory service acts as the main switchboard of the network operating system. It is the central authority that manages the identities and brokers the relationships between distributed resources, enabling them to work together. Because a directory service supplies these fundamental operating system functions, it must be tightly coupled with the management and security mechanisms of the operating system to ensure the integrity and privacy of the network. It also plays a critical role in an organization's ability to define and maintain the network infrastructure, perform system administration, and control the overall user experience of a company's information systems.