How do public key rings work in cryptographic applications? I am wondering if there are any links to articles or resources available online, or explanations that you can give, that would help me to understand the concept of a public key ring, and why I might (or might not) want to publish my public key to one or another of them.

In other words, I have a basic understanding (I hope) of how PKI works and why I'd publish my public key to a keyserver so people can find me, but I'm unclear on what these different rings mean, or what their application is.

I'm wondering what would be a good analogy for them?  Like, Google+ circles?  I am looking for a better understanding of this concept.
It's shockingly simple. It's a file. It has public keys in it.

The traditional PGP Key Ring is a sequential file with a sequential list of keys in it. It's not even a database.

Slightly more advanced key rings, such as those used in Key Servers actually use a database.