http://www.google.com/adsense AdSense for content automatically crawls the content of your pages and delivers ads (you can choose both text or image ads) that are relevant to your audience and your site content—ads so well-matched, in fact, that your readers will actually find them useful. AdSense for search allows 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.
In simple word Adsense is a revenue generating program from Google. You can earn mone by publishing google advertisements on your website. For more information checkout google adsense link provided in the above posts.
Well Google Adsense is one of the way where you can make money from it. For that you need to work hard if you really want some decent earnings! You can read more about Google Adsense and how it works here: All About Google Adsense