Click Fraud
Click fraud is perpetrated in both automated and human ways. The most common method is the use of online robots, or "bots," programmed to click on advertisers' links that are displayed on Web sites or listed in search queries. A growing alternative employs low-cost workers who are hired in China, India and other countries to click on text links and other ads. A third form of fraud takes place when employees of companies click on rivals' ads to deplete their marketing budgets and skew search results.

Human operations can be more difficult to detect because a wide network of people can click on ads from different computers across many regions, without a steady pattern. According to a report in the India Times, residents are being hired to click paid links from home, with the hopes of making between $100 to $200 per month.

In other instances, the source of bogus clicks can be much closer to home.

Joe, the chief executive of an Internet marketing company, enjoys clicking on his rivals' text ads on Google and Yahoo because his competitor must pay as much as $15 each time he does it. Eventually, such phantom clicks can add up and drain a rival's budget.

Read some more here.