The exact details of PCI are pretty complex, but, roughly the answer is that any system that touches credit card numbers has to be PCI compliant.

This means that if you take credit cards on your website, you need to be PCI compliant. But the exact requirements differ depending on your setup, and what Stripe helps you do is ensure the credit card information never hits a server owned by you. This drastically reduces the scope of your compliance to a simple questionnaire.

As for being outside the US, not quite yet. We're working hard on that front though, so stay tuned.