A few months ago, I attended a PayPal Developers Days conference in Miami. The seminar covered the various ways to integrate PayPal Pro into your web site to accept payments. The options included using Express Checkout which lets the user log on and choose a billing and shipping address and a method of payment; Direct Pay which allows a web site to accept credit cards cleared by PayPal without the customer having to have a PayPal account and Standard Checkout which redirects the customer to PayPal's site to log on and complete the payment. PayPal sends an IPN (Instant Payment Notification) message to a web service-like page on your site so you know you can proceed to fulfil the order.
I have been doing PayPal development for a long time so the test given at the seminar was trivial - a few hours of refresher using the supplied study guide were sufficient to quickly complete (and pass) the exam. Eventually, I got listed on the PayPal Directory.
I also got a nice PayPal shirt:)