Definition of CPA
CPA stands for Cost per Action in Advertising Terms. It might refer to a user putting in their email into a form in its simplest format, or it might require that they fill out a complete multi-part form full of information before you are paid.
CPA in Affiliate Marketing
CPA is also the term used with relation to the sale of Affiliate Merchandise, where you carry a link or image of a product on your site, and when the user clicks on it to purchase it, your site sends them to a merchant site to complete the transaction. Once purchased, you receive a percentage of the sale to the consumer.
The range of commissions on a CPA deal varies widely. It could be as little as 2% or as high as 50% or more. The value of the affiliate program is in the rate of payment AND the rate of conversion (which is how many people actually decide to purchase the product).