Since its introduction in 2003 Google’s Adsense program has been helping bloggers defray hosting charges and other costs related to running their blogs. Blogging can be expensive, especially when you have high levels of traffic and numerous pages. So many are turning to advertising such as Google Adsense to help generate revenue.
Google Adsense is an advertising program that is run (of course) by Internet giant Google. Google’s Adsense allows blog owners like you to sell advertising space by displaying relevant text and banner ads on your blog content  pages.
Banner ads are the most common form of online advertising displayed at the top of many blog pages. Google pays you a fee when a visitor clicks on the ad. Because the ads are targeted to what visitors are looking for on your blog, or they match the interests of the visitors your content attracts, you can earn and at the same time enhance your content pages.
Google uses its vast search technology resources to serve ads based on blog content … you can read more at How Much Does Your Google Adsense Really Make.