Months back I read an article in USAtoday.com which is a very good explanation of what google adsense has been upto.
Since its launch in 2003, the Google Adsense program has revolutionized Web publishing, turning blogs and personal websites into potentially lucrative ventures.
It is easy to join. A Web publisher or blogger simply completes an online Adsense form. Google then places ads on the site, similar to those that appear next to search results on Google. The ads are contextually matched to content on the host site, so a blog post about having a headache might attract ads for pain-relief medicine.
Anyone with a site is eligible, and Web forums are awash in success stories of small online entrepreneurs placing ads on their sites, sitting back, and watching checks from Google roll in.