Directory (databases)

Generally, a directory, as used in computing and telephony, refers to a repository or database of information which is heavily optimized for reading, under the assumption that data updates are very rare compared to data reads. Commonly, a directory supports search and browsing in addition to simple lookups.
A website which offer access to a categorized listing of other websites optimized for lookup, search, or browsing is a directory, a web directory. The Open Directory Project is an example of a web directory.
Directory technology is often used in white page applications and network information services.
WHOIS is a prominent example of directory service providing white pages.

The Domain Name System (DNS) is a prominent example of directory service providing a network information service. DNS is also an example of a distributed hierarchical directory service that only has simple lookup capabilities.

The X.500 and LDAP directory services are examples of general-purpose distributed hierarchical object-oriented directory technologies. Both offer complex searching and browsing capabilities and are used for white pages, network information services, public key infrastructure, and a wide range of other applications.