Affiliate marketing is a method of promoting web businesses (merchants/advertisers) in which an affiliate (publisher) is rewarded for every visitor, subscriber, customer, and/or sale provided through his/her efforts.

Affiliate marketing is also the name of the industry where a number of different types of companies and individuals are performing this form of internet marketing, including affiliate networks, affiliate management companies and in-house affiliate managers, specialized 3rd party vendors and various types of affiliates/publishers who utilize a number of different methods to advertise the products and services of their merchant/advertiser partners.

Affiliate marketing overlaps with other internet marketing methods to some degree, because affiliates are using the same methods as most of the merchants themselves do. Those methods include organic search engine optimization, paid search engine marketing, email marketing and to some degree display advertising.

Affiliate marketing - using one site to drive traffic to another - is the stepchild of online marketing. While search engines, e-mail and RSS capture much of the attention of online retailers, affiliate marketing, despite lineage that goes back almost to the beginning of online retailing, carries a much lower profile. Yet affiliates continue to play a fundamental role in e-retailers' marketing strategies.^[1]

Referrence:
Wikipedia

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