Acronym for Name Service Switch
The NSS is a glibc mechanism for providing multiple methods of accessing common databases such as the password or hosts database. It is commonly used to provide an interface to both local /etc/passwd (and shadow) files, and a remote authentication mechanism such as NIS or LDAP.
The databases available for configuration within the NSS are:
Most of these databases are normally only configured for flatfile access: protocols and services, for example, look up only in /etc/protocols and /etc/services. You could serve these from LDAP, or from a DBM? file, or from an SQL DataBase, or so on.