Directory services were part of an Open Systems Interconnect (OSI) initiative to get everyone in the industry to agree to common network standards to provide multi-vendor interoperability. In the 1980s they came up with a set of standards - X.500, for directory services. The lightweight protocol access protocol, LDAP, is based on the services of X.500 is based on the TCP/IP stack and has therefore become more relevant.
What distinguishes a directory server from a relational database is that in a directory, the information is generally read more often than it is written. Hence the usual database features of transactions and rollback are not implemented in a directory. Data may be made redundant, but the objective is to get a faster response time during searches.
A directory service basically maps the names of network resources to their respective network addresses. Similar to a domain name service, the user doesn't have to remember the physical address of a network resource; providing a name helps locate the resource. Each resource on the network is considered as an object on the directory server. Information about a particular resource is stored as attributes of that object. Information within objects can be made secure so that only users with the available permissions are able to access it.
A directory service defines the namespace for the network. A namespace is a set of rules that determine how network resources are named and identified. The rules specify that the names be unique and unambiguous. In LDAP, such a name, called as distinguished name (DN) is used to refer to a collection of attributes which make up a directory entry.
Novell's implementation of directory services NDS is the best breed of this technology; it supports multiple architectures among them Windows, Netware, several flavours of Unix, AS/400 and Linux and has long been used for user administration, configuration management, software management. It has superior scalability.