Where a third party advertises products or services on behalf of a retailer in return for an agreed commission for a sale or lead.
This method has proved very popular due to its' "pay for performance" model which allows a large number of affiliates to be taken on, but only rewarding those which return results.
Affiliates will usually be selected to ensure they are suitable to market the products of the retailer, e.g. a high street family grocer may not want a sex site advertising their services.
There a number of affiliate networks which aggregate many affiliate schemes so an advertiser can receive payment for their sales from one company, but deal with many retailers.