A cookie is a small piece of data that is transferred to a computer in order to mark it for a later transaction. Affiliate programs are based on cookies. The way they work is that when a user comes to your site and clicks on the link of a site you are affiliated with, a cookie is placed on that user’s computer. Then, even if they leave the site and come back a week later to make a purchase, you will get credit for the purchase and receive a commission. A merchant can make the duration of a cookie as long or as short as they want, depending on their needs and strategy. Some are a year long, other only a few days. Generally, smaller and more specialized vendors will have longer cookies, while big consumer companies like Wal-mart or Best Buy will have very short ones.