From the PayPal’s documentation:
Instant Payment Notification (IPN) is a message service that automatically notifies merchants of events related to PayPal transactions.
Merchants can use it to automate back-office and administrative functions, including automatically fulfilling orders and providing customers with order status.
developer.paypal.com/docs/classic/products/instant-payment-notification
From the Stripe’s documentation:
Use webhooks to be notified about events that happen in a Stripe account.