CPA or "Cost per Action" (also called "Cost per Acquisition) refers to a marketing system in which a merchant with a product or service pays other parties for introducing prospective customers to them and their offerings.
The distinction between CPA and general affiliate marketing (to which it bears a strong resemblance) is that in CPA, it isn't necessary for the prospective customer to actually purchase the product or service. If the prospect takes an action specified by the merchant the CPA affiliate is paid. That action may be (and often is) filling in and submitting a form, accepting a free trial, or some other way of interacting with the merchant.
Frequently, merchants place restrictions on how the CPA affiliate may acquire the leads submitted to the merchant. These restrictions differ depending on the merchant's needs.
For additional information, see in Wikipedia.
