How are postgreSQL database files structured?

Hey folks:

I am mounting the learning curve from MySQL to postgreSQL.  It seemed so
straight forward before.  On my Win98 box, everything was clearly named as
I had designated in the c:/mysql/data directory.  On the Debian server, the
/var/lib/mysql directory is similarly and intuitively laid out in an
understandable way.

But when I take a look at /usr/local/pgsql/data/base, I am baffled by its
contents.  I hear that the directory at: /usr/local/pgsql/data/base/1 is
the template1 database.  But I do not know where the template0 database
is.  And I certainly do not understand how testtest gets named 16555, much
less what the numbered directories and files subordinate to that directory
have to do with anything real in the world.

Can someone please refer me to a document (preferably off the net, as my
book buying budget is non-existent, these days) which will explain it all
for me?  I would certainly appreciate that.  Thanks.

-- Hugh Esco