A telephone service allowing one to obtain people or organisations' phone numbers by providing other details, usually their full name and address. In the US, this service is known as directory assistance.
Oftel list of UK Directory enquiries services (http://www.newdirectoryenquiries.com/local_numeric.html) (Apparently incomplete due to a charge levied by Oftel)
Directory of UK Directory enquiries services (http://www.marcuskern.com/info/118.htm)
Directory assistance (in the UK, directoryenquiries) in telecommunications is a telephone call one can make to find out a specific phone number of a residence or business.
Use of directory assistance could incur a charge when using your local, long distance or wireless carrier, now often over one dollar per call, although in some localities, laws mandate a certain number of free directory assistance calls per month.
In the UK, directoryenquiries used to be reached by dialling 192 (domestic numbers) or 153 (foreign), with the service supplied by the former monopolist, British Telecom.
Directories are a way of keeping track of all of those resources, along with their various properties.
In a directory environment, all applications would reference employee information from the same data pool, meaning that any changes made to their details would only have to be input once.
Active Directory is a pretty complex system and experts claim that you have to define your structure very early on in the process, because it is difficult to change once you have rolled it out.