Important: If you are using an API Certificate as the API Authentication Method, you will need to import a new certificate for your store by August 6, 2013. PayPal has upgraded to 2048-bit key length SSL certificates, and your certificate may be out of date. For more information, see this post from PayPal.
Some PayPal payment solutions require API credentials to establish a connection to the PayPal server. Merchants engaged in business outside of United States are required to provide API credentials if using either PayPal Express Checkout or Website Payments Pro.