Cost-Per-Action (CPA) - This is the rate/fee that is set for every action that is taken by on visitor which is promoted by an affiliate. This usually refers to, by not limited to, Pay-Per-Lead and Pay-Per-Sale transactions. CPA, as Cost-Per-Action usually corresponds to Pay-Per-Lead while CPA, as Cost-Per-Acquisition usually corresponds to Pay-Per-Sale.
Earnings-Per-Click (EPC) - This is a metric to measure the effectiveness of being able to turn a click into commissions.
Formula: EPC = (Clicks/$Commissions Earned)
* Note, come companies, like Commission Junction (CJ's) change the definition slightly. In CJ's case, EPC refers to earning per 100 clicks.
Formula: Commission Junction EPC = (1/100)*(Clicks/$Commissions Earned)