newbie DC question

From: Rob Snyder (rsnyder_at_reversedthebrokenglass.com)
Date: 12/28/04 

Date: Tue, 28 Dec 2004 02:41:26 GMT

Greetings.

I've been studying electronics on my own for a little while now, and
there's one basic concept I can't seem to get my head around. I
apologize in advance for the really basic question, but I just can't
find the answer and it's keeping me up at night.

I thought I understood DC, at least at a primitive level. In circuits
with a battery, things make sense - there is a positive and a negative,
current flows from the negative through the circuit to the positive...
makes sense.

Where I get lost is when I'm looking at a circuit with an AC to DC
transformer. Typically, the output of this, after the rectifier, is a
positive DC source, a negative DC source, and ground.

What *is* this ground thing? Does current flow from the negative output
to ground? From ground to the positive? Both? Neither? What they heck am
I missing?

Most explanations I've read just say something to the effect of "ground
being a reference, with the current being more negative or more positive
than ground". Needless to say, I can't turn that into something I
understand.

I appreciate any guidance anyone has to offer.

Thanks!

Rob Snyder