/private/var/db vs /var/db

I'm just getting to grips with how things are on Mac OS X Server vs Ubuntu Linux with which I am more familiar.

Wondering if there's a document summarising Darwin's directory structure conventions?

Specifically I am generating DKIM signing keys and would do this into /var/db on my Ubuntu server.

I have noticed /private.

What is the purpose of /private/var and how does it relate to /var?