Shell, admin, and both:
| These are primarily notes|
It won't be complete in any sense.
It exists to contain fragments of useful information.
YP (Yellow Pages), now also often referred to as NIS (Network Information Service), is an RPC-style system that lets you synchronize system configuration between hosts, and seems primarily used specifically to handle simple centralized accounts (for logins, or or just to keep UIDs and GIDs synchronized - useful in, say, a cluster or just using NFS).
YP is an extra step on top of your usual per-host management (some parts are relatively manual), doesn't scale very nicely to very large setups, and not all implementations are terribly secure, so in large setups also see other systems.
YP has a learning curve, in part because when you use it you may want to centralize another half other things.