Affiliate programs are online programs that allow owners of web sites to place links to third-party advertisers and retailers, and earn a commission for each action they refer. A simple example of an affiliate program is Amazon's program, which pays webmasters from 2-15% commissions on sales they refer of books, electronics and other items. Affiliate programs generally pay webmasters on a cost-per-sale (CPS), cost-per-action (CPA), or cost-per-click (CPC) basis, with sales tracked automatically through web-based software or a third-party service.

Today, affiliate marketing has grown to be a massive business, supported by such industry heavyweights as CJ.com and BeFree, as well as a host of smaller players. Affiliate programs continue to represent an excellent way for many web sites to boost their revenue.