LDAP
Lightweight Directory Access Protocol (LDAP) is a technology used to create directories of individuals or resources.
The LDAP standard describes how directory data should be stored in a tree structure, how it is to be queried (using a custom query language), and how an LDAP client should communicate with the directory server.
LDAP is generally used by medium to large organizations to keep a central directory of their members. LDAP distinguishes itself from relational database technology by being optimized for high-frequency access, at the cost of slower write speed. Additionally, LDAP directories tend to follow common schemas, to maximize compatibility with LDAP clients.
The most well known LDAP implementation is Microsoft's Active Directory -- though it extends the general standard.
Modern languages have libraries for communicating with LDAP directories. Care should be taken not to expose yourself to injection attacks if you are communicating with an LDAP system.