Problems with Paypal

Hi,

What I want to do is simple. I want to Paypal on my website so that after a user pay the product, I want to change role of the user to another (example from membership level 1 to level 2).

I have read all the documentation and videos, and I extract these steps:

1. – Create a test environment Paypal.
2. – Insert correctly all the parameters needed by WordPress
3. – Enter a payment button with the price and other aspects.

My question is:

Once the customer has paid in Paypal, how I can Paypal notify to my WordPress site? Is there any function avalible that hear the Paypal answer (If the user pay or not)? How is it implemented? I read something about IPN ​​messages but do not know how I do this… I’m confused with this concept.

Thanks!!