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What Is AdSense for Content?

So what exactly is AdSense for content? It's a program by which third-party websites (which must meet certain criteria to be accepted into the program) rent out space on their pages to Google. Google displays advertisements (drawn from the AdWords ad pool) in those spaces; all the site owner (known as an AdSense publisher) does is paste some Google-supplied JavaScript onto the page to display the ads. If someone visiting one of the third-party sites clicks an ad, Google shares the revenue it makes from that ad click with the site owner. It's a deceptively simple model.

Underneath that simplicity lurks some sophisticated programming, however. Not in the ad delivery itself—the JavaScript code is fairly conventional. It's the ad-selection code that is unique.

The first time the AdSense code runs on a page, Google sends a crawler (not the Googlebot crawler you might already be familiar with, but a different crawler not associated with the main Google search index) to fetch the page for analysis. Google then analyzes the content of the pages and tries to determine the keywords that are most closely related to or associated with the content. As part of this analysis, Google considers a number of factors, such as keyword density and the placement of keywords within certain tags. Google also considers its previous analysis of other pages on the same site or pages that link to the new page. (Most of the details of how AdSense works are covered in fair detail in the AdSense patent application.)

Within a few minutes (it takes between 1 and 15 minutes for Google to send its crawler after the AdSense code is placed on a new page and activated at least once), Google has compiled a list of keywords and phrases related to the page content. These are then used to search the AdWords ad pool. Only relevant ads from the ad pool are displayed on the site. Visitors are therefore more likely to click on such advertisements because presumably they're already interested in the page's topic.

Note that the ads displayed by AdSense for content can be text or image ads. Initially, Google restricted itself to serving text-only ads, but those restrictions were later dropped. AdSense publishers can choose on a page-by-page basis what kind of advertisements to serve, what format they should be (Google offers a number of different types and sizes of advertisements), and where they should be placed on the page.

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