Commission based. A percentage is paid to you every time your web site generates a sale. Whenever your visitor buys something from the merchant's site, you get a commission from it.

You may be rewarded for every sale you induce (each purchase generates a commission), or for all the purchases made for a customer along a certain period of time (the amount of sales will define your commission), it will all depend on the affiliate program you make part of.

The commission scale is variable, according to each type of affiliate program: it can be a flat commission, whatever the amount of sales you get, or the commission may slide in order to reward highly visited web sites.