Hi,
Adsense is page specific (page content targeted) text ad's run by Google.
https://google.com/adsense/
Every time a unique visitor clicks on one of the ad's from your site you get a percentage of the revenue that advertiser bids per click - Google don't publish the exact percentage but its believed to be around 30-40%.
By experimenting with my sites Adsense ad placement last month (tried at the bottom of the page, using towers right, changing the colours etc.) I managed to double my click through rate ;)