ARC is the Airlines Reporting Corporation and IATA is the Int'l Air Transport Association.
Both organizations accredit travel agents (IATAN does it for IATA). They are registered through either or both and they receive a membership number. This number can be used for various discounts. I believe that any agent can get what is called a "75AD", which is 75% off any full fare. This works out to be not much for economy (since they can just book a discounted fare, often for less) but is a huge savings for biz and first class. There might be other discounts but I am not aware of their specifics.