CPA marketing is a type of affiliate marketing. Cost Per Action simply means you get paid a commission based on an action that is set by the owner of the product. So they may set a commission of say $1.50 to get someone to fill out their email address for an offer. Or $35 to fill out a form for say a free trial of their product.

Read this article:
Affiliate Marketing Tips – How to Choose the Top CPA Affiliate Programs