Postgres Vacuum Cleaners

[Raf]

Gents and Ladies,

I've become rusty with postgres and figured that someone might be able to
shed some light.

I am trying to understand postgres' vacuum functionality.
I've started working with a DB, which I'm told uses "auto_vacuum."

Now, to me this says little about the vacuuming policy, however, I have
seen the there are some very large files under the postgres data device.
It seems that the size of these files exceeds the typical size of a
database dump, which to me suggests that there is still a physical history
of stale tuples in the DB and that it may need a full vacuum.  Googling
has led me to believe that auto_vacuum only rebuilds stats / does a vacuum
analyse.  Thus, I don't believe that this would free up space on the
device?

Now, I was hoping that someone might be able to give be a brief synopsis
of the different vacuum arguments and their functionality.  I've tried
googl'ing, but I'm hoping that this will be more meaningful.  Further, is
it correct to assume that while vacuuming the db would be taken off line,
or is there a way to expose it in a query-only mode?

The goal of my mission is to free up space and speed up queries.
Suggestions?

Many Thanks.

Raf