In CPA Cost per action you make money for any specific action. Mostly the action Involves filling of a form given by advertiser. when a visitor makes a specified action you make money for it. For example there is a offer of McDonald's $1000 deal available on a CPA network. you will display that offer On your website or blog. when a visitor of your site will click that offer and then fill out the given form to get that offer you will make money for it.
This is how the CPA works. you can promote the CPA offers not only through your blog or website but also with other allowed ways like Social media, classified sites, email marketing etc.
For each action you can make from $1 to $50 depending upon the offer that you are promoting. most offers are country based and will only earn when people from specified countries will take the specified action.
I recently made good amount of money by promoting CPA offers through Facebook ads. There are large number of good CPA networks that you can join. some of them are following: