Affiliate programs are programs in which other site owners will place links to your site and/or products on their site. If/when a visitor clicks a link from their site to your site and makes a purchase, a program of some sort will track that purchase. You will then pay a commission to the site owner that sent the customer to your site. This service can be part of your web site, but is generally better if outsourced to a company like Commission Junction (cj.com) or LinkShare (linkshare.com). Affiliate marketing companies like CJ provide the software and tools to manage your affiliate program and a pool of affiliates who might choose to sign-up as an affiliate for your company.