We need a bit more information than just "IPN."
Generally IPN stands for Instant Payment Notification and works by calling a specific page on your website with the payment details so you can record the transaction in your back end system.
Customer Pays on the Gateway Site
Gateway Provider calls IPN script
IPN sends customer/payment info to your backend system
You record the details
Customer gets redirected to your success page and the information about the transaction is viewable to the client in the client area (or however you might to do it).
IPN generally requires a specific script to be created either by you or a 3rd party; as we don't know the gateway we can't say anything more. A lot of gateway's are now supported in this regard by existing software - such as billing and automation systems - that can handle this for you.
Take a look at the billing and automation Wiki page:
Hope that helps,