CPA stands for ” Cost Per Action” which means the companies that pays the affiliate/marketer a fixed pre-defined commission for an action taken by the online user such as filling a short form such as their name, address, email, etc…..
Promoting CPA companies can be very profitable for an affiliate as the affiliate do not have to make a sale of their product in order to get commission but the affiliates get paid handsomely by these affiliate companies for a lead i.e. basic form filling or completing a survey or trying any product, downloading any software and etc.