Stands for "Lightweight Directory Access Protocol." If you want to make directory information available over the Internet, this is the way to do it. LDAP is a streamlined version of an earlier directory standard called X.500. What makes LDAP so useful is that it works great over TCP/IP networks (unlike X.500), so information can be accessed through LDAP by anyone with an Internet connection. It is also an open protocol, which means directories can be stored on any type of machine (i.e. Windows 2000, Red Hat Linux, Mac OS X).
To give you an idea of how an LDAP directory is organized, here are the different levels of a simple LDAP tree hierarchy:
Most LDAP connectivity is done behind the scenes, so the typical user probably won't notice it when surfing the Web. However, it is a good technology to know about. If nothing else, it is another term to impress your parents with.