Google’s $6 billion-a-year advertising business is at risk because it can’t be sure that anyone is 
looking at its ads. The problem is called click fraud, and it comes in two basic flavors.
With network click fraud, you host Google AdSense advertisements on your own website. Google pays you every time someone clicks on its ad on your site. It’s fraud if you sit at the computer and repeatedly click on the ad or — better yet — write a computer program that repeatedly clicks on the ad. That kind of fraud is easy for Google to spot, so the clever network click fraudsters simulate different IP addresses, or install Trojan horses on other people’s computers to generate the fake clicks.
The other kind of click fraud is competitive. You notice your business competitor has bought an ad on Google, paying Google for each click. So you use the above techniques to repeatedly click on his ads, forcing him to spend money — sometimes a lot of money — on nothing. (Here’s a funny spoof site that offers to commit click fraud for you.) Wired News: Google’s Click-Fraud Crackdown