This is the common name (for those in the
airline industry) for the Airline Tariff Publishing Company. ATPCO is an
independent company owned by 24 other
companies, (mainly airlines, but the
list includes
Fed Ex).
ATPCO is a central clearinghouse and publisher of information related to the pricing of airline (and other modes of transportation) tickets, throughout the world. A large percentage of the world's airlines file their prices (fares) and restrictions (rules) with ATPCO. ATPCO also collects tax and surcharge information. All of this is repackaged by ATPCO and sold to various CRSes, travel agents, and the airlines themselves.
ATPCO was recently mentioned in the news when Southwest Airlines used ATPCO to stop various web sites (but mainly Orbitz) from getting information about their prices in an automated fashion (so they could be searched via Orbitz's search engine). Southwest used a Category 14 restriction that basically said "all travel must have commenced by yesterday in order to use this fare". They applied this rule to the overwhelming majority of their fares.
ATPCO does have competitors, or at least companies that do similar things, especially for international airlines.
There is a wealth of information about ATPCO at its home page, http://www.atpco.net. This page goes into ATPCO's history and products, among other things.