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.
Always make affiliate links nofollow. Google’s goal is to provide its users with the best possible search results to their search queries. One of the main ways they figure out which webpages are the best is through links. The Google bots crawl the web, reading text. When they come to a link, it’s like an open door. They follow the link (walk through the door) and explore the site you linked to.
Affiliate marketing is also called "performance marketing", in reference to how sales employees are typically being compensated. Such employees are typically paid a commission for each sale they close, and sometimes are paid performance incentives for exceeding objectives. Affiliates are not employed by the advertiser whose products or services they promote, but the compensation models applied to affiliate marketing are very similar to the ones used for people in the advertisers' internal sales department.