Definition: Google AdSense is a fast and easy way for website publishers of all sizes to display relevant Google ads on their website’s content pages and earn money. Because the ads are related to what your visitors are looking for on your site — or matched to the characteristics and interests of the visitors your content attracts — you’ll finally have a way to both monetize and enhance your content pages.
FirstClickSEO Commentary: It’s also a way for website publishers to provide Google web and site search to their visitors, and to earn money by displaying Google ads on the search results pages.