Google's ad services let anyone put advertisements on their Web site. They can be anything from generic banners all the way to contextual ads that show offers related to content on the site. In addition to site ads, site owners can also purchase AdWords, which are purchased keywords in search terms. When a user buys one of these words, a link to their site will show up in the sponsored area to the right of a Google search using that word.
One of the reasons Google's ads have become so successful is that they can easily be integrated into many sites and blogging services with very little knowledge of coding or advertising. Google has also integrated the ad statistics into their analytics tool, to let people keep track of who is clicking them.
Winner: Google AdWords (Adwords.Google.com)