CPA means ‘cost per action’ or ‘click per action’ depending on who you talk to.
The big difference between this and something like Google Adsense is that you don’t get paid when someone just clicks on the advert on your site, the visitor has to do something on the site that you send them to before you ‘earn’ your money. That might not sound as good as getting paid per click, and it wouldn’t be if the payouts were the same, but the payouts are much higher.
And if you’re sending targeted traffic getting the visitor to take action isn’t all that hard.
1. The landing pages you send people to are nearly always excellent.
2. The offers are very good.
3. Often all the visitor has to do before you get paid is to leave their name and (verified) optin email address or/and phone number.
You can make a lot of money by just getting people to sign up for free offers.
Look at this one from the Azoogle Ads network (offer removed) but it did pay you $28 when people send off for a free trial of an anti-aging product. And how about sending people to an offer to get 250 free business cards from Vistaprint? I’ve been getting my business cards from Vistaprint for years and they’re great quality. I just wish I’d known that I could have been making $14.40 a time for telling people about it 
Note: these are OLD offers and they may no longer exist, or the prices may have changed.
Azoogle is now part of Epic Direct.