Google is the biggest company in the online advertising market. And it is interesting that Google shares its revenue to other companies and individuals. The Google Adsense is that program.
Adsense is really an affiliate programs, it is special. You do not need to make sure the product/services will be bought. All you need to do is putting ads code into your website. Ads will be showed and you will be paid for clicks on those ads. This way is a kind of "selling ads space on your website".
How to make money with Google
It's simple and free! Just sign up for a Google Adsense account, then get the ads code and put into your website. That's all! But, be careful. It's not really easy to get real money from Google. You must read the policy careflly, and follow it stricly to avoid the account disabling. Read more about Google Adsense here