default domain by app

Is there a way to set, in the configuration, a default domain for my
app?

The whole domain system eludes me, and I can't find much in the way of
docs on it.  (probably just not looking in the right place, i'm sure) 
If you could point me to that, that would be a great help, as I'd like
to understand more than I'd like my problem fixed.  If I had a clue, I
might quit bothering you with stupid questions so you could actually do
some development yourself.

thanks in advance
gene