That's pretty much the gist of it mate. Adsense and other networks will pass a load of surveys, pin submits, double opt ins and other methods to the ppd sites. We register and upload files and get people to download, we then get a % of whatever the ppd site earns.
Cpa networks are harder to get into as you are cutting out the middle man (ppd) and in turn receiving a bigger % of the $$$. But!!! To join a cpa now days you have to have a phone call of them and answer a few questions and so on, if you answer the phone and your still in your teens then 9/10 chance your voice hasn't dropped which means they can tell your still young hence why a few people get turned down (my thoughts anyways) There are plenty of other reasons for being turned down i.e lowsy site, didn't do to well on the phone call and so on.
So we head over to a ppd site were you don't have to have a phone call or a site for that matter! More people get accepted into a ppd than cpa networks.
I dunno if I sumed it up correctly as I'm new to IM aswell but that is what Ihave pretty much worked out. Maybe a more experienced member can elaberate on it a bit more
Edit: I'm a member of sharecash and just been accepted into fileice. Allready I can see the bigger payments of Fileice