working with users

Ok, I am a newbie, but I simply cannot find anything in the
documentation so I hope you all are kind to me...

I have installed Postgres 7.3.4 (will reinstall once I get OpenSLL
compiled into it). I created the postgres user, and the first
database. As the postgress user, I am able to run psql and access
the database that I created. I have been able to run the create
user command.

But how do I see what users exist for the database? Because I was
the user postgres when I created the database, does that mean there
was a postgres user added during creation? If not, what user am I
when I run psql as postgres? If I am postgres, what is the password
to the database and how do I rest it?

Now I am not a DBA, but I have been around enough databases to know
that most of them have their own system tables. Does postgres have
system tables? If so, where do I find a list of them and their
usage?

Sam