A few comparison terms just to be sure.

Hi everyone.
For a long time I've been an Oracle developer and administrator but recently
I became seduced by what I read about Postgresql. So I downloaded and
installed a server and I'm reading the docs right now, ready to start
practicing.
Just to feel at home, I would like to make a few comparisons between oracle
and postgresql and I hope somebody can confirm if I'm right or not.
The questions:
1- What I know as an oracle database (a group of schemas), in postgresql is
called a cluster. Is that right?
2- what I know as "schema", in postgresql is called a "database"?
3- Every user is the superuser of his own schema/database but is there a
SYS/SYSMAN/SYSTEM (DBA) user who has access to all "databases"? I place
where I can log in and control what everyone is doing?
4- What in Oracle is the SYSTEM/SYSAUX schema (where all oracle's own
objects reside) is the "postgres" database (I didn't find this schema
although I read about in the docs: chapter 16.2)?

I know these are pretty simple question and I'm really thankful in advance
for any comments.

Regards.

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