Google's revenue model is pay-per-click advertising. The advertisements that Google has on its own websites and the advertisements that it places on other websites with Adsense provide Google more than 90% of the revenue it earns. Using complex algorithms Google makes the advertisements as relevant as it can to the person visiting the Google website or any other website where Google has placed the ads.
The advertisers do not have to pay Google for visitors seeing their advertisement but instead only if a person clicks on the ad and is led to the advertiser’s website. With Adsense Google can place advertisements on web sites it does not own and in return the website owner is paid a percentage of the revenue Google earns from the advertiser.
Just advertisements bring Google over $8 billion every year in profits.