Google AdSense is a program run by Google Inc. that allows publishers in the Google Network of content sites to serve automatic text, image, video, and rich media adverts that are targeted to site content and audience. These adverts are administered, sorted, and maintained by Google, and they can generate revenue on either a per-click or per-impression basis. Google beta-tested a cost-per-action service, but discontinued it in October 2008 in favor of a DoubleClick offering (also owned by Google). In Q1 2011, Google earned US$2.43 billion ($9.71 billion annualized), or 28% of total revenue, through Google AdSense.
An open source license is a copyright license for computer software that allows the source code to be freely, that is by anyone for any purpose, used, modified and shared. This allows end users to review and modify the source code for their own customization, curiosity or troubleshooting needs.