I always refer people to this page here as it describes in easy to understand terms on how credit cards are processed.

There a literally hundreds if not thousands of Acquirer's and yes they do have their share of tech related issues and do go down from time to time such was the case yesterday.

These days the downtimes are a lot less for many of the reasons Martin alluded to....but it only takes one malfunction to really mess things up.

When I am back at work on Monday, I will see if anyone knows what transpired with First Data...