Instant Payment Notification (IPN) is PayPal's interface for handling real-time purchase confirmation and server-to-server communications. IPN delivers immediate notification and confirmation of PayPal payments you receive and provides status and additional data on pending, cancelled, or failed transactions.
# Customize your website's response to customer purchases in real-time
# Track customers via IPN "pass through" variables
# Deliver access keys for software downloads and other digital goods
# Automate your fulfillment operations