The main way of earning revenue with AdSense is by displaying Google ads on your site.
With AdSense for content, these ads you display on your site can be either cost-per-click (CPC) or cost-per-thousand-impression (CPM) ads. For CPC ads, you'll generate earnings when your users click on the ads. For CPM ads, you'll generate earnings every time the ad appears to a user viewing your site.
You won't, however, receive revenue from user searches through your AdSense for Search box. When users perform searches on your site, but choose not to click on the ads on the search results pages, you will not receive any earnings.
No, participation in AdSense is free. Even better, Google will pay you for clicks or impressions on the Google ads you'll display on your site. For more details on the revenue you can generate with AdSense, read our entry on earning with AdSense.
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