An affiliate program is when retailers pay you a commission for generating traffic to their store. For example, if a user reaches the iTunes store through your recommendation, you will earn 7% of any purchase they make.
In order to identify your commissions, you should add a unique affiliate ID to all links leading to retailers such as iTunes. If you enter your affiliate ID in your Linkfire board settings, Linkfire will automatically make sure that any sales generated by your campaigns will contribute towards your commission payout.
You can read more about (and apply for) the iTunes affiliate program here: http://www.apple.com/itunes/affiliates/