This is nss-pam-ldapd which provides a Name Service Switch (NSS) module that allows your LDAP server to provide user account, group, host name, alias, netgroup, and basically any other information that you would normally get from /etc flat files or NIS. It also provides a Pluggable Authentication Module (PAM) to do authentication to an LDAP server.

This is implemented using thin NSS and PAM modules which delegate to a dedicated service (nslcd) that queries the LDAP server with persistent connections, authentication, attribute translation, etc.