Network Information Service (NIS) is a directory service for Unix originally developed by Sun Microsytems. NIS was first called Yellow Pages (YP), but the name was changed due to trademark issues. NIS is a client-server protocol, allowing Unix or Linux servers to act in a manner generally analogous to a Windows domain controller. NIS does not scale well, however, and other directory services such as LDAP are more commonly used in current deployments.
References:
http://www.freebsd.org/doc/handbook/network-nis.html
http://www.diablotin.com/librairie/networking/firewall/ch08_15.htm