AdSense is an ad serving program run by Google. It is "Pay Per Click" and "Contextual".
Website owners can enrol in this program to enable advertisements on their sites.
They are available as image, more recently video but far more famously for the text ads.
Google utilizes its search technology to serve ads based on website content (contextual), the user's geographical location, and other factors.The source of all AdSense income is the AdWords program
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".

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