Google AdSense

Google AdSense

"Google uses a number of technologies to display relevant ads to visitors, whether indexing the content of Web sites, geographic location and other factors, including big brand specials. This guarantees to advertisers looking to reach audiences, while delivering useful ads to users.
All resources come from AdSense investment Advertisers Google AdWords, which is a complex model of instant auction where competing ads more performance for the Editor."
Source: Wiki