What exactly is the ACH (Automated Clearing House) network and how does it work?

I found an excellent response regarding ACH on Google answers.  A shorter response on ACH is also hosted on Google's site.

NACHA, the National ACH Association, has recently put together a new web site, electronicpayments.org,  outlining Direct Deposit, Direct Payment, and check conversion.

If you ever get involved with writing or maintaining computer code that creates and manipulates ACH files, you *must* get your hands on NACHA's operating rules and guidelines.  This book, which is updated by NACHA annually, not only explain the rules, but provides the file layouts for coding ACH files.  Purchasing a copy of the book also allow you access the ACH rule book on-line, via achrulesonline.org.