The major credit card companies created the PCI Data Security Standard (PCI DSS) to make sure merchants adopt critical security measures. There are 12 requirements for meeting the PCI DSS, broken into 6 groups:

Build and Maintain a Secure Network

REQUIREMENT 1: Install and maintain a firewall configuration to protect cardholder data

REQUIREMENT 2: Do not use vendor-supplied defaults for system passwords and other security parameters

Protect Cardholder Data

REQUIREMENT 3: Protect stored cardholder data

REQUIREMENT 4: Encrypt transmission of cardholder data across open, public networks

Maintain a Vulnerability Management Program

REQUIREMENT 5: Use and regularly update anti-virus software

REQUIREMENT 6: Develop and maintain secure systems and applications

Implement Strong Access Control Measures

REQUIREMENT 7: Restrict access to cardholder data by business need-to-know

REQUIREMENT 8: Assign a unique ID to each person with computer access

REQUIREMENT 9: Restrict physical access to cardholder data

Regularly Monitor and Test Networks

REQUIREMENT 10: Track and monitor all access to network resources and cardholder data

REQUIREMENT 11: Regularly test security systems and processes

Maintain an Information Security Policy

REQUIREMENT 12: Maintain a policy that addresses information security