A directory is a specialized database specifically designed for searching and browsing, in additional to supporting basic lookup and update functions.
A directory is defined by some as merely a database optimized for read access. This definition, at best, is overly simplistic.
Directories tend to contain descriptive, attribute-based information and support sophisticated filtering capabilities. Directories generally do not support complicated transaction or roll-back schemes found in database management systems designed for handling high-volume complex updates. Directory updates are typically simple all-or-nothing changes, if they are allowed at all. Directories are generally tuned to give quick response to high-volume lookup or search operations. They may have the ability to replicate information widely in order to increase availability and reliability, while reducing response time. When directory information is replicated, temporary inconsistencies between the replicas may be okay, as long as inconsistencies are resolved in a timely manner.
There are many different ways to provide a directory service. Different methods allow different kinds of information to be stored in the directory, place different requirements on how that information can be referenced, queried and updated, how it is protected from unauthorized access, etc. Some directory services are local, providing service to a restricted context (e.g., the finger service on a single machine). Other services are global, providing service to a much broader context (e.g., the entire Internet). Global services are usually distributed, meaning that the data they contain is spread across many machines, all of which cooperate to provide the directory service. Typically a global service defines a uniform namespace which gives the same view of the data no matter where you are in relation to the data itself.
A web directory, such as provided by the Open Directory Project <https://dmoz.org>, is a good example of a directory service. These services catalog web pages and are specifically designed to support browsing and searching.
While some consider the Internet Domain Name System (DNS) is an example of a globally distributed directory service, DNS is not browseable nor searchable. It is more properly described as a globally distributed lookup service.