Adsense,

How AdSense works
The webmaster inserts the AdSense JavaScript code into a webpage.
Each time this page is visited, the JavaScript code creates an IFrame with a src attribute set 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, advertisements are served for those keywords based on the AdWords bidding system. (More details are described in the AdSense patent.)
For site-targeted advertisements, the advertiser chooses the page(s) on which to display advertisements, and pays based on cost per mille (CPM), or the price advertisers choose to pay for every thousand advertisements displayed.
For referrals, Google adds money to the advertiser's account when visitors either download the referred software or subscribe to the referred service. The referral program was retired in August 2008.
Search advertisements are added to the list of results after the visitor performs a search.
Because the JavaScript is sent to the Web browser when the page is requested, it is possible for other website owners to copy the JavaScript code into their own webpages. To protect against this type of fraud, AdSense customers can specify the pages on which advertisements should be shown. AdSense then ignores clicks from pages other than those specified.

Abuse
Some webmasters create websites tailored to lure searchers from Google and other engines onto their AdSense website to make money from clicks. These "zombie" websites often contain nothing but a large amount of interconnected, automated content (e.g., a directory with content from the Open Directory Project, or scraper websites relying on RSS feeds for content). Possibly the most popular form of such "AdSense farms" are splogs (spam blogs), which are centered around known high-paying keywords. Many of these websites use content from other websites, such as Wikipedia, to attract visitors. These and related approaches are considered to be search engine spam and can be reported to Google.

A Made for AdSense (MFA) website or webpage has little or no content, but is filled with advertisements so that users have no choice but to click on advertisements. Such pages were tolerated in the past, but due to complaints, Google now disables such accounts.

There have also been reports of Trojan horses engineered to produce counterfeit Google advertisements that are formatted looking like legitimate ones. The Trojan downloads itself onto an unsuspecting computer through a webpage and then replaces the original advertisements with its own set of malicious advertisements.


Criticism
Due to concerns about click fraud, 'Google AdSense' has been criticized by some search engine optimization firms as a large source of what Google calls "invalid clicks", in which one company clicks on a rival's search engine advertisements to drive up the other company's costs.

To help prevent click fraud, AdSense publishers can choose from a number of click-tracking programs[citation needed]. These programs display detailed information about the visitors who click on the AdSense advertisements. Publishers can use this to determine whether or not they have been a victim of click fraud. There are a number of commercial tracking scripts available for purchase.

The payment terms for webmasters have also been criticized. Google withholds payment until an account reaches US$100, but many micro content providers[citation needed] require a long time—years in some cases—to build up this much AdSense revenue. However, Google will pay all earned revenue greater than US$10 when an AdSense account is closed.

Google came under fire when the official Google AdSense Blog showcased the French video website Imineo.com. This website violated Google's AdSense Program Policies by displaying AdSense alongside sexually explicit material. Typically, websites displaying AdSense have been banned from showing such content.

It has been reported that using both AdSense and AdWords may cause a website to pay Google a commission when the website advertises itself.