Most of our ad units have "site" type. They are used for banners built into a site code and shown at sites. We can check the quality of traffic that's generated by these ad units, so they normally earn more if they pass the check.

"App" type is for ad units that you want to place somewhere else: mobile applications, browser extensions, etc. We cannot check quality of such traffic, so it comes as anonymous in our system. So these ad units usually earn less.  

"Affiliate" type is for ad units, that are not supposed to show advertisements. They exist only as an account in our network and are used to give affiliate links and earn fees from campaigns created via these links. Imagine, you don't have neither a site, nor a mobile app, nor a browser extension, but you've got a neighbor who needs advertising. You create an affiliate ad unit, give your neighbor a link, he creates a campaign and you get 1/2 of his fees. 

"Site" and "app" ad units also can be used to create affiliate links though.
Was this article helpful?
Thank you!