When Google launched Adsense in 2003 they kept quiet about how they split the revenue from advertisers. Seven years later they have finally revealed how the revenue is split.
Google revealed on their AdSense blog stating, "Today, in the spirit of greater transparency with AdSense publishers, we’re sharing the revenue shares for our two main AdSense products — AdSense for content and AdSense for search."
They say that 68% of all revenue from AdSense for content goes to the publisher. AdSense for content is the advertising that you see on websites that use Google's AdSense service to generate revenue.
AdSense for search partners earn 51% of the revenue generated from ads. AdSense for search is the advertising you see next to your search results when using a sponsored search box. Firefox takes uses Google's AdSense for search by providing the search box built into the browser. When someone clicks on an advertisement after searching Google with the Firefox search box Firefox gets 51% of that revenue generated.
Google says the portion of the revenue they keep for content based ads "reflects Google's costs for our continued investment in AdSense — including the development of new technologies, products and features that help maximize the earnings you generate from these ads. It also reflects the costs we incur in building products and features that enable our AdWords advertisers to serve ads on our AdSense partner sites" While the portion they keep for search based ads goes into research and development for their core search and AdSense integration.