A fraudulent click!!!

A fraudulent click, surely, should be a click which is action purely for the purpose of generating income for the publisher.

It's certainly perfectly valid to click on an ad if you think you might purchase something. And what if the site doesn't sell anything at all? Does that mean that content advertisers get free ads? Yes, it's perfectly permissible to click on an ad out of interest and then choose not to buy. Why on earth would you define a click as fraudulent purely because an advertiser fails to close a sale once the visitor arrives at their site.

How can they have trace any invalid clicks or click fraud?
Google has a sophisticated system to detect the invalid clicks, and result in termination of the cheaters’ account. But, how can Google do that? The ways that Google depends on to detect whether the clicks on your site are invalid or valid.

IP Address
It is the easiest way to detect if those clicks on your ads are originated from the same IP Address as the one used for accessing your AdSense account, your account is flagged.

Click Through Rate (CTR)
Normally, Click thru Rate should not excess 10%. Otherwise, Google will flag your account. For your information, normal CTR should ranges from 0.5% – 10%.
Physical Location
Google has good tracing software and technology. They can trace traffics origin down to the small town.

Cookies
Google has set cookies on your computer. They can trace these cookies and see whether they originate from the same computer.
Click Pattern
It is also suspicious when people click on their clicks and then run away immediately. But normally, people will surf for a while inside your pages and then click on the ads they want, why this computer / IP address / person is so trigger-click-happy on this particular website but never click on the ads on other sites? And why is it that people accessing these sites direct key-in URL or from bookmark tend to be very active ad-clickers compared with those referred from search engine or other sites?

Hardware address
Your modem, your LAN card, routers has a serial number which act like a fingerprint? These can be used as tracing evidence by Google.