Network Information Service (NIS) is a client-server directory service protocol used for managing network resources. It allows systems to share information about users, hosts, and services across a network, making it easier to manage large numbers of computers. NIS was originally developed by Sun Microsystems in the 1980s and is commonly used in Unix and Linux environments. It simplifies administrative tasks by centralizing data, such as user accounts and passwords, which can be accessed by multiple machines, enhancing efficiency and consistency in network management.