Arbitrage is used by some Internet entrepreneurs to take advantage of the price difference between some advertising keywords in AdWords and AdSense.
The basic system is that someone buys a inexpensive AdWords campaign, such as "cheap widgets" for ten cents. The ads direct anyone clicking on them to a Web page that is optimized for a more expensive keyword, such as "expensive widgets" for five dollars per click. If even a fraction of people visiting "expensive-widgets.com" click on the ads, the arbitrageur has turned a reasonable profit.
Although there is nothing explicitly forbidden about using arbitrage to profit from AdSense, it is often a technique employed by low quality content producers, and Google has shut down some very profitable accounts that were using this technique.
