NIS (Network Information System) is a system which is mainly used to give users in a Unix environment the same configuration on every computer of the network. It stores configuration files on servers in so called NIS-maps. So if the user has an account in this network, (s)he can login from every computer. If NFS is used in conjunction, one gets the same environment (as long as the same Unix version is used, if not, it's a bit more complicated).
In a NIS environment there are three types of computers: The master-server contains the NIS database and provides the NIS functionality. The slave-server contains a copy of the NIS database and provides the same functionality as the master. A slave is present in every subnet and they help the master by taking load away from them. Clients are normal Unix stations, which use the services provided by the servers.

The configuration file /etc/nsswitch.conf must be present on every client. NIS decides using this file, from where to get configurations. A short example from this file:
hosts: files dns nis
passwd: files nis
protocols: nis[NOTFOUND=return] files
Explanation: The rows show the search order. For example, if a login is not found in the local passwd, NIS is used. The last line shows a more complicated version. For protocols is searched using NIS. If nothing is found, the search will be aborted. The local file is only used if NIS is down.

Finally some NIS commands:
As NIS was formerly known as Yellow Pages the commands begin with yp:
ypwhich - shows which NIS server is used
ypcat - shows the table of the NIS server
ypmatch - searches for an entry in a NIS table
yppasswd - changing the password
ypset - set your NIS server