ACH is an acronym for a method of moving funds from one bank account to
another bank account via a network between participating financial
institutions.  This funds movement is accomplished via a NACHA defined file.
It's nothing more than a text file in a specific format, where each record
is a line in a "fixed-width" format.

Any bank can provide you with a booklet that defines these file formats.  My
latest copy is the "2007 ACH Rules - Corporate Edition".  This booklet has,
maybe, 150 pages in it that defines all aspects of NACHA (National Automated
Clearing House Association) activity.

Incoming and outgoing file formats are the same and are governed by the
above NACHA rules; one banks incoming is another banks outgoing.

If you have specific questions I'm sure we can answer them.

Hope this helps.

Bill