Like any blog, my blog relies on the small amount of income I make via the Google banner ads. When a web publisher such as myself adds Google Adsense banners to a website Google places banner ads on the website based on the content that Google detects. Therefore if you have a boating blog you might see some ads featuring boating manufacturers or fishing and tackle companies. If the topic of your blog is knitting you will likely see Google ads that feature hobby and crafts companies. And if the topic of your blog is about dogs you will likely see Google ads from carpet cleaning companies. In short, Google guesses what ads should be displayed on a website based on what it thinks the audience would be interest in.