Affiliate marketing has increased in prominence with the internet age. Amazon popularized the practice by creating an affiliate marketing program where websites and bloggers put links to the Amazon page for a product being reviewed or discussed in order to receive advertising fees when a purchase is made. In this sense, affiliate marketing is essentially a pay for performance marketing program where the act of selling a consumer on a product is outsourced across a potentially vast network.
Win-win-win. The advertiser wins because they only pay when a purchase is made (as opposed to the shotgun approach of paying to advertise to the masses and waiting for a small percentage to actually buy). The affiliate wins because they make money while providing helpful advice. The customer wins because they get a trusted recommendation for something they might not otherwise have known about.
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.