Affiliate Marketing is an Internet-based marketing practice in which a business rewards one or more affiliates for each visitor or customer brought about by the affiliate's marketing efforts. It is an application of crowdsourcing.

The Affiliate Marketing industry has four core players at its heart: the Merchant (also known informally as 'Retailer' or 'Brand'), the Network, the Publisher (also known informally as 'the Affiliate') and the Customer. The market has grown sufficiently in complexity to warrant a secondary tier of players, including Affiliate Management Agencies, Super-Affiliates and Specialized Third Parties vendors.

Affiliate marketing overlaps with other Internet marketing methods to some degree, because affiliates often use regular advertising methods. Those methods include organic search engine optimization, paid search engine marketing, e-mail marketing, and in some sense display advertising. On the other hand, affiliates sometimes use less orthodox techniques, such as publishing reviews of products or services offered by a partner.

Affiliate marketing—using one website to drive traffic to another—is a form of online marketing, which is frequently overlooked by advertisers. While search engines, e-mail, and website syndication capture much of the attention of online retailers, affiliate marketing carries a much lower profile. Still, affiliates continue to play a significant role in e-retailers' marketing strategies.