Google AdSense is the program where you can host Google pay-per-click ads on its Web site. When someone clicks on an ad, you earn money. Simple as that.
Is it really so easy to earn money with Google AdSense as many would lead you to believe? Now that I've become very involved with the programme management Google AdSense for my clients, I can see that's not all play and no work.
Unfortunately, many people bought into the idea that there is a lot of easy money to do and now are discovering that they are not doing nearly as much as they thought they would. In addition, Google has a large amount of prohibited practices when it comes AdSense, and also many webmasters know that Google violates policies after the fact - often because they never bother to read the AdSense policies in the first place.
Source : http://dhuwuh.blogspot.com/2008/08/adsense.html