AdSense is an advertisement application run by Google. Website owners can enroll in this program for free to enable text, image, and more recently, video advertisements on their websites. These advertisements  are administered by Google and generate revenue on either a per-click or per-impression basis.

AdSense for content automatically crawls the content of your pages and delivers text and image ads that are relevant to your audience and your site content. 

Currently, AdSense uses JavaScript code to incorporate the advertisements into a participating website. If the advertisements are included on a website that has not yet been crawled by the Mediabot, AdSense will temporarily display advertisements for charitable causes, also known as public service announcements (PSAs). (The Mediabot is different from the Googlebot, which maintains Google's search index.)