PCI DSS Compliance is a set of regulations to ensure security of online transactions with credit card companies and banks.
If you're planning to take credit card details directly on our servers as opposed to a 3rd party gateway such as PayPal Express or Google Checkout, your bank or card company will most probably ask for PCI compliance statements.
We periodically check our server security to ensure compliance but you will also have to perform this test and submit your own reports directly to your bank.
For this purpose we currently recommend HackerGuardian from Comodo which is the least expensive system we've found to perform automatic checks without being too troublesome.