What is "CPA"?
"CPA" stands for "Cost Per Action" where an action can be anything from a click to a lead or a sale. For all the long-time Clickbank affiliates that advertise with PPC campaigns, the excellent news sits with the leads especially. The sales work in much the same way as with CB where you get paid a commission for selling whatever product. The lead offers on the other hand usually require no purchase by the visitor.
There are a few "types" of lead offers I've found.
Type 1: "email submit" offers, as I've heard them be referred to. As the name implies, these pay out just for someone submitting their email address (similar to all those "get an ipod for free" looking sites, but also used to build email lists). These usually pay out ~$1-$2.
Type 2: "Personal Databases", like dating services etc. These require the user to sign up (all the ones I've seen are free for the user) to whatever service, and the pay out is about $4-$6.
Type 3: A common lead program is a variety of insurance companies, mortgage companies, car quotes, and everything similar. In this case you send the visitor to a "sales" page and payout occurs upon completion of some contact information. If you think of it in terms of CB sales, it almost feels like cheating. If the user is interested enough to learn more (with no obligation to pay), you get paid. These are usually 1 to 2 page forms. From what I've found these have payouts from $10-$30.
Type 4: These are the subscription services like VOIP etc. These can pay out up to and over $100, but require a sign-up (so kind of like a sale).
What remains are the ones that pay per click and per sale.
The PPC ones are simply that: each time someone clicks on the link/banner, you get paid ~$.01-$.05. If you get a lot of traffic on your website you might be interested. The good thing with these is that often if they follow your link and that results in a LEAD, you get paid a considerably bigger sum, and possibly even bigger with a sale (similar to some of the offers Shareasale has).
The PPS (pay per sale) ones are straightforward and all us CB guys know how it goes, but here it can get a bit interesting. There's lots of offers on here that don't require any specific product to be sold, just whatever product off a website (for example a coffee shop website). These are a lot like Linkshare.
Why promote CPA networks?
ROI
Most importantly: ROI! My ROI is absolutely ridiculous with CPA offers when compared to Clickbank even at its best. Simply put, these convert like crazy, and if you find the right offer with the right payout, your home free. As an example, I started slowly promoting some Azoogle offers starting on the 1st of March on a $4 a day budget, and by the 8th or so I had made over $400. Often I hadn't spent close to my $4 a day (low CPC costs), and so on an investment of ~$12-$15 I made just over $400. That kind of ROI is what made me jump head long into CPA networks.
Affiliate Managers
Second, and also very importantly, the affiliate managers (your contact with the CPA network) are amazing! They will more than gladly help you with any questions over the phone or AIM, they'll gladly tell you what offers are converting the best (because the more you make the more they make), and they'll even boost your payout if you ask them (example: I went from $40 payout to $42.50 on an offer, and from $1.20 to $1.70 on some email submits). Even better, you often don't even have to ask them. My AM told me out of the blue after I asked her some questions that she'll boost the payout on any offers I want to promote.
Summary: Fantastic communication with the network!
These are just my opinions so far in the 25 days or so since I've joined CPA networks. Perhaps ClayTrainor and hgarcia can correct me on anything I may have inadvertently misrepresented or will just add anything I missed.
Hope that helps a bit.