AdSense and SEO resource

AdSense is an ad serving program run by Google. Website owners can enroll in
this program to enable text, image and, more recently, video advertisements
on their sites. These ads are administered by Google and generate revenue on either a per-click or per-thousand-impressions basis. Google is also currently beta-testing
a cost-per-action based service.
Google utilizes its search technology to serve ads based on website content, the user's geographical location, and other factors. Those wanting to advertise with
Google's targeted ad system may sign up through AdWords. AdSense has become a popular method of placing advertising on a website because the ads are less intrusive than most banners, and the content of the ads is often relevant to the website.
It currently uses JavaScript code to incorporate the advertisements into
a participating site. If it is included on a site which has not yet been crawled by the Mediabot, it will temporarily display advertisements for charitable causes known as public service announcements (PSAs).
(Note that the Mediabot is a separate crawler from the Googlebot that maintains Google's search index.)