Google AdSense is a free, simple way for website publishers of all sizes to earn money by displaying targeted Google ads on their websites. AdSense also lets you provide Google search to your site users, while earning money by displaying Google ads on the search results pages.
How does it work?
Choose the type and placement of ad units to be displayed
Specify where you want ads to appear
Choose what types of ads can compete for those slots
Highest-paying ads display
Advertisers bid on your inventory in a real-time auction