The Network Information Service, or NIS, is a client–server directory service protocol for distributing system configuration data such as user and host names between computers on a computer network. Sun Microsystems developed the NIS; the technology is licensed to virtually all other Unix vendors.
The Network Information Service (NIS) provides a simple network lookup service consisting of databases and processes. It was formerly known as Sun Yellow Pages (YP). The functionality of the two remains the same; only the name has changed. Its purpose is to provide information, that has to be known throughout the network, to all machines on the network. Information likely to be distributed by NIS is:
login names/passwords/home directories (/etc/passwd)
group information (/etc/group)
host names and IP numbers (/etc/hosts)
So, for example, if your password entry is recorded in the NIS passwd database, you will be able to login on all machines on the net which have the NIS client programs running.
Linux machines can take full advantage of existing NIS service or provide NIS service themselves.