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.
Be sure to check what kind of customer support you can expect from your affiliate program once you have signed up. Do your research online and if possible, speak to other sellers using the program to get their thoughts. Can you speak to someone via phone or Skype or do you have to wait 72 hours for email responses? Be clear on this because trust me, you will need support at one point or another.