It is a google advertising component, which allows blogger to display google ads links,text and image (known as ''adsense for content'') in their own blog or blogs to earn dollars. And now google expanded the opportunities in making money by adding google adsense for search, for feeds and for domain. But the most effective way in earning dollars is the adsense for content. The adsense for content will be displaying google ads' links, texts and images depending on what country your viewer comes from, eg. When the viwer of your blog comes from the US, then it will only display US based products and if the viewer comes from India, then it will only display India based products, thus making the google ads more relevant to the viewer, it varies from country to country, and you will be earning dollars per click of the google ads displayed on your blog. Need not for those products to be sold. Adsense for seach can earn you dollars by putting these custom search gadget into your blog and through it's link you can earn. Adsense for feeds can earn you dollars through subscription to your blog feeds, it is made possible by feedburner. Adsense for domain can earn you money, if you have an idle or parked domain and apply that domain for adsense for domain, then make the necessary changes with domain's DNS settings, then your domain will show ads and links instead of the usual content, so with those links and ads, you can earn dollars from it. Google adsense makes money making a lot more easier for bloggers, so apply for google adsense account now.