Google Adsense advertising program
Google AdSense is currently the largest online advertising service. Advertising type is CPC (cost per click).
How it works
Webmasters go to google adsense page and fill a form to apply for an AdSense account; you also have to mention your web site URL and if your site meets the Google AdSense quality standards then Google will send a note telling you that you ware accepted.
Then you login to your AdSense account and basically copy a html code, you paste that html code inside your site code and in around 10 minutes you should see the ads appear on your site.
When someone clicks an ad then you get paid. The amount you receive per click depends on the category (automotive, real estate, news, technology, etc) you can get paid more or less for each click, other factors that affect the amount you receive per click are visitor's country, ad relevance compared to your site and your web site quality score, from what I noticed the quality score is based on conversions you generated, if users keep clicking ads but they never buy anything form that site then you will have a low quality score and receive less per click.
A medium of a good payment per click is probably around 0.10 - 0.20 $ for web sites from various categories. There are situations when you get paid few dollars for a single click.
For a bigger monthly payment you would need to get highest payment per click and highest click through rate (what percent of visitors click the ads). There are many thinks you can do to optimize the amount you receive per click and the click through rate; for example placing more ads on same page usually cause you to get paid less because when having many ads, the ads that pay more can load in an area of page layout where nobody noticed, and the ad that loads in the most visible place can be one that loaded later and pays less per click, so my opinion is to only place one or two ads on one page and make sure they are in the most visible location, next to navigation buttons, next to page title, etc.