A directory is a repository of information about objects, which is organized in such a way that it is easy to find the information on a specific object. A common example is a telephone directory, where information (address and telephone number) is stored about people and companies. Another example is an address book for an e-mail system, where e-mail addresses, and optionally other information such as telephone numbers, are stored for people.
On computer systems, directories can store information about computer resources, such as printers or shared disks. For example you could use a directory to find out where the nearest color printer is located. In a WebSphere MQ application a directory can be used to provide the association between an application service (such as accounts-receivable processing) and the queue to be used for messages requiring that service (possibly identified through the queue name and its host queue manager name).
Directories are implemented as client-server systems, where the directory server holds all the information and answers requests from clients. The clients could be user-interface programs, which provide the information directly to the user, or application programs which need to locate resources to complete their work. A Directory Service comprises the directory server, administrative programs, and the client libraries and programs which are needed to configure, update, and read the directory.