The Payment Card Industry (PCI) Data Security Standard was jointly developed by Visa and MasterCard in December 2004, to simplify compliance for merchants and payment processors. The PCI standard has 6 core areas and 12 requirements, covering a broad scope of best practices for perimeter security, data privacy, and layered security.
Any business that deals with credit card data -- whether they're processing, transmitting, or storing it -- needs to abide by the PCI standard or risk having their credit card transaction privileges revoked.