Click fraud exists in many forms. Automated bots, competitors and even large click farms are rumored to exist in third world countries.

But each type produces a common result: erroneous click traffic that advertisers have to pay for.

The most common types and motivations of click fraud are:

Competitive fraud: As its name implies, competitive click fraud is most often seen but not limited to heavily competitive categories in which a competing advertiser seeks to remove a competitor by endlessly clicking on the competitor's search terms. The end result is a forced removal either by budgetary attrition or by the advertiser deeming the keyword or phrase ineffective.

Affiliate or contextual fraud: Many websites generate revenue from listings placed on each of their sites: each time a user clicks on a listing, the website gets paid. So, to put it simply, an unscrupulous website owner might arrange for a lot of clicks on the site ads in order to increase revenue from content listings.

Impression fraud: Searches are conducted repeatedly on a term but no clicks are registered. This drives the clickthrough rate for that paid search listing down. At that point, either a search provider's automatic tool will disable the search term (due to a low clickthrough rate) or the advertiser will do it manually. This clears the way for another advertiser's paid search listing to move up in the rankings. Since there is no money changing hands, and technically there is no click action, this type of fraud is difficult to detect.

Automated fraud: Software applications that verify links can contain robots or crawlers that automatically enter sites or follow links; sometimes these robots can inadvertently click on paid listings.

Next: Identifying click fraud