Google.com earns revenue by other website owners to advertise their site on search result pages that is managed through a program called google AdWords. Whereas, you working as Publisher and webmaster can earn a share of the revenue that Google earns from AdWords by placing these same text ads on your sites and blogs. You earn because you are helping Google to advertise and google pay you a percentage of earning. All this is managed through a program called AdSense.I think every website owner should be involved in this to a make decent money with AdSense. Suppose, if you earn some dollar in a month, it will cover all of the costs for your web sites. That is why, this program is getting so popular, people are creating websites just to display the ads. This program is completely free to join and you can also use it on multiple websites/blogs and there is no limit to the amount you can earn.
"How Much Can You Earn With AdSense?"
Google gives you a share/percentage that depends on how much advertisers by Adwords are paying Google for the particular ad. You will earn a share of that amount that may be from 2 cents to $15 per click, it all depends on highly competitive keywords that are searched. Advertisers generally pay more for popular terms because they are searched for more. Even though Google will not reveal how much you are earning for each ad that is clicked from your site, you can still login to your account at any time and see the total amount of revenue you've generated that day, week, month, year, etc. The amount you'll earn also depends largely on the amount of targeted traffic you receive to your own site, how well the ads match your audience's interests, the placement of the ads on your pages.