In short
What is the difference between /etc/ldap.conf and
/etc/ldap.conf is not part of OpenLDAP and is used by LDAP PAM module and Nameservice switch libraries for authentication or name service resolution.
/etc/openldap/ldap.conf ?
/etc/openldap/ldap.conf is an OpenLDAP configuration file, used by OpenLDAP clients. This file is sufficient to query an LDAP server. -Amit