What is AdSense?

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.)
Many sites use AdSense to monetize their content and some webmasters work hard to maximize their own AdSense income. They do this in three ways:
They use a wide range of traffic generating techniques including but not limited to online advertising.
They build valuable content on their sites which attracts AdSense ads which pay out the most when they get clicked.
They use copy on their websites that encourage clicks on ads. Note that Google prohibits people from using phrases like "Click on my AdSense ads" to increase click rates. Phrases accepted are "Sponsored Links" and "Advertisements".
The source of all AdSense income is the AdWords program which in turn has a complex pricing model based on a Vickrey second price auction, in that it commands an advertiser to submit a sealed bid (not observable by competitors). Additionally, for any given click received, advertisers only pay one bid increment above the second-highest bid.