OVERVIEW
Google uses 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.

Currently, AdSense 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). (The Mediabot is a separate crawler from the Googlebot that maintains Google's search index.)

Many sites use AdSense to monetize their content. AdSense has been particularly important for delivering advertising revenue to small sites that do not have the resources for developing advertising sales programs and salespeople. To fill a site with ads that are relevant to the topics discussed, webmasters implement a brief script on the sites' pages. Sites that are content rich have been very successful with this advertising program, as noted in a number of publisher case studies on the AdSense site.

HOW ADSENSE WORKS?

• To put ads on a web page, the webmaster inserts JavaScript code into the page.
• Each time a page with an AdSense tag is visited, the JavaScript creates an iframe and sets its "src" attribute to the page's URL.
• For contextual advertisements, Google's servers use a cache of the page to determine a set of high-value keywords. If keywords have been cached already, ads are served for those keywords based on the AdWords bidding system. More details are described in the AdSense patent.
• For site-targeted ads, the advertiser chooses the page(s) to display ads on and pays based on CPM (cost-per-thousand-impressions, or the price advertisers choose to pay for every thousand ads displayed).
• For referrals, Google adds money to the advertiser's account when visitors either download the referred software or subscribe to the referred service.
• Search ads are added to the list of results after a user performs a search.
• Since the JavaScript is sent to the web browser when the page is requested, it is possible for other site owners to copy the JavaScript into their own web pages. To protect against this type of fraud, AdSense customers can specify the pages on which ads should be shown. AdSense then ignores clicks from pages other than those specified.