The Network Information Service (NIS) provides a simple
network lookup service consisting of databases and processes. 

A good howto: 
hxxp://www.debian-administration.org/articles/36

NIS is a service that provides any user on that network 
with the same working environment irrespective of the 
system on that network which has been used for login 
purpose.
For example if NIS server is set up in a single system and 
configured to hold user accounts and their passwords and 
access information. Then any user on that network can login 
to his/her account that is set up in the NIS server from 
any system (with nis client running) on that configured 
network. This gives a look and feel that the user is logged 
into his/her own system. But actually its the account on 
the NIS server that is mounted on the local sytem on user 
login .

nis stands for network information service is used for 
sharing the files across the network nis is used to share 
the files in network its used in linux machines only

the configuration file of nis is vi /etc/exports

first create a folder ex: mkdir /sales
than open configuration file than write
ex: /sales 192.168.1.0/255.255.255.0(rw,sync)
that means share the file across the network than save and 
exit . if u want to see the file is shared or not command 
is showmount -e server ip address to acess the shared files 
in client machine first create a dir ex: /sales
than mount the server shared dir the command is mount 
192.168.1.0:/sales /sales
permanently mounting open configuration file vi /etc/fstab
than write /sales /sales nfs default 0 0 than save and exit

answer 3 for nfs server

NIS is a service that provides any user on that network 
with the same working environment irrespective of the 
system on that network which has been used for login 
purpose.
For example if NIS server is set up in a single system and 
configured to hold user accounts and their passwords and 
access information. Then any user on that network can login 
to his/her account that is set up in the NIS server from 
any system (with nis client running) on that configured 
network. This gives a look and feel that the user is logged 
into his/her own system. But actually its the account on 
the NIS server that is mounted on the local sytem on user 
login .

NIS is used for mapping between host name and IP address 
for a wide range of information. The clients that get 
mapped information from the server consists of user 
database and authentication information across a network.