Click fraud generally comes in two varieties that I'll call "buy" and "sell." An example of buy-side click fraud would be my little sister religiously clicking on every Google ad on this page (What? We have no Google ads (AdWords)?) in the mistaken belief that doing so would make me some money. It is mass clicking by a single person without an intention to actually buy or even to gain information. Sell-side click fraud would be one advertiser clicking on the ads of a competitor with the intention of costing that competitor money without increasing their sales. Both types of click fraud ought to be detectable, and in fact, Google says it already detects the 10 percent or so of clicks that are fraudulent (Business 2.0 magazine says it is more like 30 percent), and adjusts the bill before the advertiser even knows what is happening.