Google AdSense

Google AdSense was the first major contextual advertising program. A large part of Google's earnings come from its share of the contextual advertisements served on the millions of WebPages that run the AdSense program. Website owners can sign up for AdSense to enable text, image and even video advertisements on their sites. Revenue is generated on either a per click (the advertiser pays search engines and other Internet publishers for a single click on its advertisement that brings one visitor to  its website) or per impression (an online advertisement impression is a single appearance of an advertisement on a web page) basis.

AdSense has become a popular method of placing advertising on websites because the advertisements are not as intrusive as banners, and the content of the advertisements is generally relevant to the website. The main reasons Websites have for using AdSense is to help their sites content alone generate revenue as they typically lack the means for developing advertising sales programs and sales people.

AdSense works providing webmasters with JavaScript code that, when inserted into WebPages, displays relevant advertisements from the Google inventory of advertisers. Because the advertisements are more targeted, they are more likely to be clicked, which generates revenue for the owner of the website, as well the server of the ad itself. For contextual advertisements, Google's servers use a cache,

(a large storage of relevant data), of the page to determine a set of high-value keywords. If keywords have been cached already, advertisements are offered for those keywords based on AdWords bidding system.

For site-targeted advertisements, the advertiser chooses the page(s) to show the ads on, and then pays based on a Cost Per Mile, or CPM (the publisher has to only load the advertising on his website and show it to his visitors in order to get paid commission), or the price advertisers choose to pay for every thousand advertisements displayed. For referrals, Google would add money to the advertiser's account when visitors either download the referred software or subscribe to the referred service, however this system was discontinued in August of 2008.

AdSense has attracted its share of criticism over the years. The main area of concern involves its susceptibility to click fraud. Some search engine optimization firms claim that it is the source of what are called "invalid clicks", i.e. one in which one company clicks on a rival's search engine advertisements to drive up the other company's costs. Google claims that to avoid invalid clicks that they daily remove publishers from their partner network. AdSense publishers can choose from a variety of click-tracking programs available for purchase that display detailed information about the visitors who click on the AdSense advertisements. Publishers can use this to determine whether of not they have been a victim of click fraud.

The payment terms for webmasters have also been criticized. Google will withhold payment until an account reaches US $100, but some micro content may require a long time to amass this amount of AdSense revenue, years in some cases. However, Google will pay all earned revenue greater than US $10 when an AdSense account is closed.