A directory service (DS) is a software application — or a
set of applications — that stores and organizes information
about a computer network's users and network resources, and
that allows network administrators to manage users' access
to the resources.
Additionally, directory services act as an abstraction
layer between users and shared resources.
A directory service should not be confused with the
directory repository itself; which is the database that
holds information about named objects that are managed in
the directory service. In the case of the X.500 distributed
directory services model, one or more namespaces (forests
and trees of objects) are used to form the directory
service.
The directory service provides the access interface to
the data that is contained in one or more directory
namespaces. The directory service interface acts as a
central/common authority that can securely authenticate the
system resources that manage the directory data.
Like a database, a directory service is highly optimized for
reads and provides advanced search possibilities on many
different attributes that can be associated with objects in
a directory.
The data that is stored in the directory is defined by an
extendible and modifiable schema. Directory services use a
distributed model for storing their information and that
information is usually replicated between directory servers.
