The terms of an affiliate marketing program are set by the company wanting to advertise. Early on, companies were largely paying cost per click (traffic) or cost per mile (impressions) on banner advertisements. As the technology evolved, the focus turned to commissions on actual sales or qualified leads. The early affiliate marketing programs were vulnerable to fraud because clicks could be generated by software, as could impressions.
This is the most popular payment offered by most of the affiliate programs. Under this program, the affiliate marketers earn commissions from the merchant each time they will send a client to the merchant website, and that client makes an actual purchase. Various affiliate programs offer a specific percentage of the sale as commission while others will pay you a fixed rate for every sale.