Affiliate marketing is a very popular practice on the Internet. Under this method you have a merchant that is ready to let other people (the affiliates) sell directly or not its products and services, in substitute for a fee. Sometimes this style of advertising is also called CPA (cost per action) or CPL (cost per lead) based.
Affiliates marketing can send prospective customers to the merchant with some tools, from banners to text links and merchandise reviews.
Look this one, in order to find suitable affiliate programs you can turn to individual company and publishers like Dreamhost.com and SEOBook.com, or join affiliate marketplaces and networks.