PayPal Pro implements 2 separate payment methods:
1) PayPal Direct pay
2) PayPal Express Checkout
Express Checkout is similar to paypal standard in user experience, but is very different in the actual processing.
Paypal Direct payments is the payment method where the user fills in the checkout information, including Credit card number on your site, and the paymnet is processed in real time without going to the paypal site.
With Express checkout, the user goes to the paypal site to set the payment details. They are directed back to your site to confirm the purchase. if the user decides to confirm, the transaction is processed in real time, and the order details are stored in the database if the transaction is successful.
Paypal standard similar, but the transaction is not processed in real time. the transaction is placed in a queue to processed on a first come first served basis. With Paypal standard an IPN Script should be used to update the order status based on the success or failure. The user experience will never see the success or failure since it is not processed in real time.