Definition of CPA in online advertising
If you as an advertiser gets a deal where you can buy pure CPA, you are in good fortune. Of course if the CPA price is good. The reason is that you minimize your own risk with CPA:
If you are selling a product for 1000 USD, and your costs are 800 USD for each sold product, all you need is a CPA lower than 200 USD to make it to the plus side. If the CPA is 50 USD, your revenue will be 150 USD for each sold product.
No cure, no pay.
It would be tempting to ask for a CPA at only 1 USD, so you'll get 199 USD in your own pocket. But then the publisher or ad network will have a marginal revenue on your campaign and would probably prioritize other competing campaigns instead.
So the key to a well functioning CPA deal is to find the right CPA for both the buyer and the seller.
If the publisher or ad network doesn't deliver any actions or acquisitions, you don't pay anything either.
See also the definition of eCPA.
I also recommended using the CPM calculator to make sure you get your numbers right.