If you are a merchant with an eCommerce shop, it is important to remain PCI compliant.
PCI stands for "Payment Card Industry" and they want to make sure that visitors who shop in eStores have their credit card information securely handled.
Some of the other requirements of PCI compliance are ...

• if you are a merchant who accepts credit card information directly on the site, then you must use a SSL certificate
• never store CVV info
• and other items

More info:

PCIComplianceGuide.org

PCI compliance a requirement for certain PayPal services