Website Payments Standard has no monthly charge and you pay transaction fees on sales. You need a premier or business account to use and you interact with the PayPal engine using regular HTML forms. Customers are not required to have a PayPal Account. The customer is redirected to PayPal to enter in their financial information, and proceed to make the payment. Once the payment has been made, they are redirected back to your site
Website Payments PRO costs $30/month plus transaction fees. You need a business account and you interact with the PayPal engine via API calls. Pro is designed for your customer to have the option to pay with credit card and stay on your site. The customer completes the transaction on your site, they are not redirected to PayPal to complete the payment.
Express Checkout requires customers to have a PayPal account. Express Checkout uses both HTML & APIs.