Click fraud is an illegal practice that occurs when individuals click on Web site click through advertisements (either banner ads or paid text links) to increase the payable number of click throughs to the advertiser. The illegal clicks could either be performed by having a person manually click the advertising hyperlinks or by using automated software or Online bots that are programmed to click these banner ads and pay per click text ad links. Research has indicated that click fraud is perpetrated by individuals who use click fraud to increase their own personal banner ad revenues and also by companies who use click fraud as a way to deplete a competitor's advertising budget.
See "How Affiliate Marketing Works" in the "Did You Know..." section of Webopedia.
Also see Refer-It.com, a directory of referral & affiliate programs for e-commerce, featuring information on over 6,500 affiliate programs.