Google Adsense is a CPC (cost-per-click) advertising program that allows publishers (anyone wanting to put ads on their websites) to insert a small amount of HTML into their sites and have ads appear that are targeted and relevant to the content of the site.
The good news is that Adsense is about the easiest way for bloggers or webmasters to start making money with their sites. After installing the ad code in your site, the Adsense spiders will crawl your site to see what all your pages are about.
They then check their inventory of ads and put related ads next to each of your articles. This works out well for everyone as someone reading an article on gardening is much more likely to buy gardening tools than someone who is reading an article about car tires.