Just to kick things here off.
LDAP is the Light-weight Directory Access Protocol. It is a standardized network protocol, database model, and set of tools etc. See the RFCs [1], Luke. It is used extensively to store information used for logging in to computers, the web and its applications, and other non-Web apps. It brings together all of the information all the apps need and stores it in a database for high-performance access by client apps.
LDAP is powerful and with power comes complexity. There are plenty of books and online documents but occasionally I/we stumble on something that might want to be put somewhere. Like here.