An affiliate program provides sites with an opportunity and a financial incentive to sell other companies' products or just display their ads. While the affiliate receives no payment for just displaying the ads, every item sold via the affiliate's links brings in a percentage of the sale. These affiliate and associate plans offer little risk to the advertiser, since they only pay on business generated. Partly for this reason, such programs are popular on a wide variety of sites, especially smaller ones. Consider the implications of affiliates for site content.
Many sites have lists of recommended books and reviews.
This kind of program certainly provides an incentive to write favorable reviews. So, when looking at reviews, product evaluations, and related Net content, check to see if the site is participating in an affiliate relationship with the seller. Anyone could link to a specific book's page at Amazon. Other programs might use a numeric code or other addition. Try shortening the URL to see if the connection will still work.