IPN, or Instant Payment Notification, is a service provided by PayPal that allows merchants to receive real-time notifications about transactions. When a customer makes a payment, IPN sends an automatic message to the merchant's server, confirming the details of the transaction. This helps businesses keep track of sales and manage their inventory efficiently. IPN is particularly useful for online businesses that need to automate their payment processing. By integrating IPN into their systems, merchants can streamline order fulfillment and ensure that they are promptly informed of any payment-related issues. This enhances the overall efficiency of online transactions.