It does what it says on the tin, really. It works similarly to PayPal - but that was probably just the version I worked with, I'm sure they'd have a Protx Direct-like version too. The only thing I can say about it is that I prefer the Protx admin pages. That, and Protx have *loads* of documentation and example code
EPDQ works very much like PayPal IPN. You have a 'callback' script which it sends details to on successful payment, then that script validates the transaction and you update your order to reflect whether payment went through OK. The callback script can sit in a directory secured by a .htaccess user/password thingy, and you set the user/password in the EPDQ admin pages.