AdSense

AdSense is a popular ad serving program run by Google.  Third parties pay Google to advertise their website URL, then website owners join AdSense in order to deliver those same URLs via text or image advertisements on their website.  Google distributes the ads to websites with matching content, thus creating highly targeted marketing.  For example, an AdSense publisher who displays info on their website about cars will automatically receive Google ads also related to cars.  Any time a visitor to the website clicks on one of the advertisements, the AdSense publisher generates revenue which Google shares between themselves and the website owner (at an undisclosed percentage) and paid by Google every 30 days.