NIS (Network Information Service) (formerly YP).

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