How to list all databases?

Hello,

I'm super new to PostgreSQL, but not SQL or RDAs in general.  I have
inherited a system where no one knows anything about it.  I'd like to
start by just finding out what databases are on it, and also what users
are created.

What I really need is a good reference.  The only I have, PostgreSQL by
Korry Douglas and Susan Douglas is pretty good but it is more of a
tutorial based learning tool.

Anything you can tell me would be appriciated.

Thanks,
Mr. Char-Lez Braden