As a website owner you would definitely think of getting some additional revenue from it. Your site is doing well for your products, now you want to have some extra earning from it. How to achieve this ?
Google AdSence can be the answer for your question.

An advertiser sign up with Google for AdWords system and creates one or more advertisements that they want to pay to have appear either on Google’s search result pages or on “content pages”. Those content pages are Web sites run by individuals and organizations that include AdSense.
Here advertisers don’t have to pay for their adverts show up on either Google or separate content sites, but instead pay when the ads are clicked by customers, what we call “pay per click” or PPC. A percentage of the amount that the company pays Google for displaying the ad is then shared with the AdSense-enabled web site owner.
Suppose an advertiser is wiling to pays $1.00 for a visitor to their through AdWords, then Google will pay you certain amount (certain percentage : Google haven’t specified it yet ).

Signing up Google AdSense can be considered as an fast an easy way to earn money by displaying relevant Google ads on your site.
All displayed ads are related to what your visitors are looking for on your site — or matched to the characteristics and interests of the visitors your content attracts — you’ll finally have a way to both monetize and enhance your content pages and will earn if they click on them. For getting a good income your site must have a huge traffic and your visitors should clicks on these adverts.
You have choice for locating these adverts within your page by placing special AdSense code into your page, you can also specify color and sizes also for ads.
It is the simple way for earning without negotiating with your advertisers, no more bills for anyone and no headache for payments.

Not every site is approved for AdSense program. Google has their own terms and condition.