Lightweight Directory Access Protocol (LDAP) is essentially a network based resource database/dictionary. An LDAP server maintains information regarding resources (who, what, where and etc.) as well as access control (who or what can access what else). One may see LDAP as NIS/NIS+ on steroids, or Windows Active Directory on a diet.
Although LDAP sounds like a self-contained protocol, it actually relies on many other protocols and services. For example, LDAP can depend on Kerberos for security, but it can also rely on TLS (previous known as SSL) for the same purpose. LDAP also uses Berkeley Database (BDB) as a data storage backend.
Like Kerberos, one can use LDAP for authentication purposes (along with many other operations!). However, like Kerberos, LDAP does not have any direct relationship with the usual log in authentication mechanisms.