Payment Data Transfer (PDT) is one of two methods by which a web site may retrieve payment information from PayPal in real-time, i.e. immediately after a payment is completed. The other method is Instant Payment Notification (IPN).
PDT is accomplished via data passed back to the merchant's web site by the customer's browser after completing the transaction. This obviously poses the risk that the customer may cancel out before this happens, leaving the merchant's site unaware that the transaction has taken place. IPN prevents this problem by directly notifying the merchant's web site of the transaction as soon as it occurs (i.e. PayPal's server contacts the merchant's server without involving the customer).