A directory service, in the technical sense, is very much like a directory service in the real world. A real-world directory service lets you look up a telephone number when you know someone’s name and location. In the same way, directory services on computers let you look for other computers, e-mail addresses, files and folders, and many other objects and attributes.
For instance, when your computer asks for a login ID and password, it will check them against a directory and then look in that directory or a related one to see what kinds of access rights you have in that computing environment. In modern networked environments, even printers and hardware devices are objects in a directory.
For a directory service to look up information it has to be stored in a standardized format, the most widely used are based on X.500 with its DAP and LDAP specifications.