what is (CPA):

 Cost Per Action (CPA) Marketing is a affiliate model where a commission is paid when a user takes a specific action. These actions include filling out a form, getting a quote, signing up for a trial, or making a purchase 

its somehow complicated but you will understand it

How you can earn money :

you searh for offers and signup and take 

To promote the products of others (whether companies or individuals) using your own link called Referral Link or Tracking ID or any other name, the important thing is that it refers to you, and here you alone, and it enables the employer to follow visitors coming through you, especially those who have completed the sale, so you should As your own, you should not use any other link to prove that you are entitled to the commission, if the sale is completed.

If any of your visitors complete the intended sale (or take a specific action, such as registering on a site, or filling out a questionnaire, or otherwise) through your link, they are followed up by the owner of the original (advertiser) site and accordingly you will receive your commission. That commission can be a percentage of the value of the product (as you all do clickBank) or a fixed amount determined by the advertiser (as MaxBounty does)

In this way, the advertiser and publisher , both of them, benefit. The advertiser gets a lot of sales for his/her products/services, while many publishers (or affiliates) promote their products, increasing their income with the commission they receive for the promotion of this product/service. Moreover, the advertiser will only pay a penny to the publisher if the sale is completed, there is no risk in it such as PPC advertising campaigns such as those carried out by large companies such as Google.

For this reason, most product builders produce their own Avalet program, offering special links to their sites where they advertise their products.

Best website to look for offers :

 http://www.affpaying.com/

this site help you in searching for offers and you can choose the offer accordin to its review

 https://www.offervault.com 

good luck in making money