NIS/YP is used for keeping a central directory of users, hostnames and most other useful things in a computer network. For example, in a common UNIX environment, the list of users (for authentication) is placed in /etc/passwd. Using NIS adds another "global" user list which is used for authenticating users on any host.
Sun licenses this technology to virtually all other Unix vendors.
As the name "Yellow Pages" is a registered trademark in the United Kingdom of British Telecommunications PLC for their (paper) commercial telephone directory. Sun changed the name of their system to NIS, though all the commands and functions still start with "yp".