Yellow Pages (YP) was invited by Sun Microsystems. The name has been changed into Network Information Service (NIS) because YP was already a trademark of the british telecom. So, when I'll talk about NIS you'll know what I mean. ;-)
There are quite some configuration files on a unix-system, and often it's desired to maintain only one set of those files for a couple of hosts. Those hosts are grouped together in a NIS-domain (which has nothing to do with the domains built by using DNS!) und are usually contained in one workstation cluster.
Examples for the config-files shared among those hosts are `/etc/passwd', `/etc/group' and -- last but not least --- `/etc/hosts'.
So, you can "abuse" NIS for getting a unique name-to-address-translation on all hosts throughout one (NIS-)domain.
There's only one drawback, which prevents NIS from actually being used for that translation: In contrast to the DNS, NIS provides no way to resolve hostnames which are not in the hosts-table. There's no hosts "one level up" which the NIS-server can query, and so the translation will fail! (Suns NIS+ seems to take measures against that problem, but as NIS+ is only available on Solaris-systems, this is of little use for us now.)
Don't get me wrong: NIS is a fine thing for managing e.g. user-information (`/etc/passwd', ...) in workstation-clusters, it's simply not too useful for resolving hostnames!