Google adsense

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If there is anybody who knows about google adsense very well please answer this. I have registered in google adsense. And it is now terminated and they told because of invalid clicks. I have a friend maintaining an internet cafe. I asked him to visit my site regularly. I think he was the only person who clicked on google ads. My question, what is the INVALID CLICKS they mentioned. Is that because all clicks came from same machine. I want to know exaly the wrong thing I have done. Please explain me.