I'm always surprised that iTunes affiliation is not very well known. It's in fact the base of a lot of business model: Apps Gratis, Free App the Day etc... Assuming you're sending a customer to the App Store from your app or your website, by any means (link to a free app, link to a paid app etc...), you're becoming an App Store affiliate and get a percentage of everything that buys this customer for 72 hours. I think this percentage is something like 5%...

Affiliation is not limited to Apple App Store, affiliation exists also on Amazon for instance, but not on Google Play, for the moment.

It's important to note that you can becomes affiliate even if your send a customer to the App Store with a free app (for instance Angry Birds Lite), and that at any time there is only one affiliate by device: that's why any apps recommendation will send you push notification each day to remain the current affiliate. And affiliation is not directly managed by Apple but by LinkShare for US, and TradeDoubler for Europe.

A last thing, doing affiliation is really easy:

https://itunes.apple.com/us/app/meon/id400274934?mt=8 => a link to my app without any affiliation

http://itunes.apple.com/us/app/meon/id400274934?mt=8&par... => a link to my app with Linkshare identifier (partnerId=30) and my company identifier (siteID=pPE4bVE6DtE)