What's AdSense?

Google AdSense is the program that can give you advertising revenue from each page on your website—with a minimal investment in time and no additional resources.

AdSense delivers relevant text and image ads that are precisely targeted to your site and your site content. And when you add a Google search box to your site, AdSense delivers relevant text ads that are targeted to the Google search results pages generated by your visitors’ search request.

source : http://www.google.com/services/adsense_tour/index.html