The Payment Card Industry Data Security Standard (PCI DSS) is a set of requirements designed to ensure that ALL companies that process, store or transmit credit card information maintain a secure environment. Essentially any merchant that has a Merchant ID (MID).
The Payment Card Industry Security Standards Council (PCI SSC) was launched on September 7, 2006 to manage the ongoing evolution of the Payment Card Industry (PCI) security standards with focus on improving payment account security throughout the transaction process. The PCI DSS is administered and managed by the PCI SSC (www.pcisecuritystandards.org), an independent body that was created by the major payment card brands (Visa, MasterCard, American Express, Discover and JCB.).
It is important to note, the payment brands and acquirers are responsible for enforcing compliance, not the PCI council. A copy of the PCI DSS is available here.