This site uses AdSense as a means to generate a bit of revenue to cover costs. The way it works is advirtisers place ads in the AdWords program, and then opt to have their ads show via the content network. Google will then use its vast knowledge of sites and their relation to each other to place contextually relavent ads on a site that is using AdSense. If a user click on an AdSense ad then the site owner and google will share the revenue from the AdWords advirtiser.
Below is an example of AdWords as presented through AdSense.