Network Information service (formerly YP) is a means of distributing
information to a group of machines. The NIS master holds the
information tables and converts them into NIS map files. These maps
are then served over the network, allowing NIS client machines to get
login, password, home directory and shell information (all the
information in a standard /etc/passwd
file). This allows users to
change their password once and have it take effect on all the machines
in the NIS domain.
NIS is not at all secure. It was never meant to be. It was meant to be handy and useful. Anyone that can guess the name of your NIS domain (anywhere on the net) can get a copy of your passwd file, and use "crack" and "John the Ripper" against your users' passwords. Also, it is possible to spoof NIS and do all sorts of nasty tricks. If you must use NIS, make sure you are aware of the dangers.
There is a much more secure replacement for NIS, called NIS+. Check out the NIS HOWTO for more information: http://metalab.unc.edu/mdw/HOWTO/NIS-HOWTO.html