Google AdSense is a free program that allows anyone with a website to display relevant Google ads and earn cash.
"Display targeted Google ads on your website's content pages and earn from valid clicks or impressions. AdSense gives you access to Google's vast network of advertisers, so you can show ads that are suited to your audience's interests."Adsense Basics
If you already have a website, why not earn a little additional income using Google Adsense. It's simple and free to set up. Just place some code on any page you want to use for ads. Adsense will use the content of your pages to display ads that your visitors will be interested in.
You can customize the size and position, the font and color, and even the types of ads that will be displayed. Take a look at the left border of this page and you will see what your ads can look like.