One of the questions asked most often by merchants considering PayPal as a payment processor is, "How will I know when the customer pays?" If a merchant is employing any sort of automation or digital fulfillment, the question becomes, "How will my site know when the customer pays?" Obviously, since customers must leave your site to complete payment at the PayPal web site, your site (or its database) won't know when your customer has paid until it has been notified by PayPal.
To that end, PayPal has developed two technologies for developers: Instant Payment Notification (IPN) and Payment Data Transfer (PDT). These technologies notify the merchant's web server when payment has been attempted, whether or not it was successful, and details about the sale.