This article is from the LAN Mail Protocols FAQ, by John Wobus firstname.lastname@example.org with numerous contributions by others.
LDAP is a protocol (the Lightweight Directory Access Protocol) being
incorporated in some clients as an Internet way for the client to get
information about e-mail addresses from a server, i.e. to give you the
capability to type in someone's name and have the mail client software
retrieve the address from a server-based directory. LDAP also has
other uses. There are plans to incorporate LDAP clients into some IMAP
and POP clients. LDAP is essentially an Internet-based, simplified
X.500-like protocol and one of the original intentions of its creators
was to be gatewayed to X.500, thus giving relatively simple Internet
clients access to X.500 servers. Both LDAP and X.500 provide a method
for naming, retrieving, and searching fields in a directory, but do
not define the field-names or what is supposed to go in the fields.
Thus server/client interoperability requires further conventions.