Using PayPal IPN (Instant Payment Notification)
PayPal IPN (Instant Payment Notification) is used to provide a means of PayPal communicating back with your application or website to automate the payment cycle when selling something. It will allow you to finalize transactions withing your system.
Essentially, when someone uses a PayPal payment button, such as a "Buy Now" or "Add to Cart" data is transmitted to PayPal so that they can process payment for you. Once this is completed, your PayPal account can be configured to post back information to your website so that you can finalize things and track the results.
This is a very common setup of many websites and we will walk through it a bit here.
Useful Videos from YouTube regarding this subject:
From Abdul Khan - His accent is not too bad and he has a very good grasp on the subject. In this video he talks about the integration using PHP and MySQLi.